In vanilla BTS, I would say that
Serfdom is the least useful Civic (besides the initial ones like Barbarism). It's not so much that it's
bad by itself but, as others have said, you have this tremendous opportunity cost by forgoing either Slavery or Caste System. But if your leader is Spiritual, you can switch into Serfdom briefly during those moments when neither Slavery nor Caste System is needed, so that you can grow your cities while improving your Worker speed.
Are there any civics that you rarely ever touch? Perhaps there are mods available to rebalance some of the vanilla civics?
The mod AdvCiv (and K-mod) balances Serfdom in what I think is a very clever way: in addition to the normal benefit to Worker speed, it additionally gives Farms and Plantations +1

while reducing the output of Towns by -1

. I think that it's clever for many reasons:
1. It prioritizes Farms, which makes sense in terms of the focus of real-life serfdom-based societies, as well as the icon.
2. Both Slavery and Caste System require a certain amount of surplus

to be effective. On maps with little

Resources and generally just crappy terrain, you don't really have that luxury. But on those maps, you often can end up with a very large number of Farms, something like 50+ sometimes. In this case, the number of Farms greatly outnumbers the number of Towns, so by running Serfdom, you end up with rather significant amounts of extra

.
3. This extra

is oftentimes important in large, sprawling kinds of empires, so that you don't go broke.
4. On those

-poor maps, spreading irrigation with Civil Service and making a lot of Farms is critical. But, this requires a lot of Workers. So, by switching into Serfdom, you can embark on your great irrigation projects more quickly with the Worker speed boost, while
also getting +1

on every single Farm you make. It works out really well.
5. During Golden Ages, each +1

becomes +2

, which is very nice. Normally during Golden Ages, you want to switch into Pacifism and starve your cities by using Specialists instead of working tiles, in order to make the most out of the Great Person points

. But sometimes, even during Golden Ages, you just really need the

. So, that's another consideration, especially if you have so little

to hire Specialists even for 8 turns, if you lack the Pyramids for Representation, etc.
The end result is that on sprawling maps with lots of Desert, Plains, and Tundra--which is to say, those resembling Russia--this form of Serfdom becomes pretty nice. Here you can see a map where the AdvCiv/K-mod Serfdom was very beneficial. Little food, huge irrigation chains, tons of Farms, not many Towns.

I think it's a shame that Firaxis didn't think of this Farms/Plantations +1

, Towns -1

idea when Civ4 was in development.
I used to think that
Mercantilism was a bad Civic, but it's really grown on me. Here are some of my reasons why Mercantilism is actually pretty good:
1. If you have the Pyramids -> Representation, then you can run a free Scientist most everywhere. This extra 6

can easily trump the

coming in from Foreign Trade Routes

, especially when it's earlier in the game and most cities are fairly small. Foreign

are not that good in many cases: just do a quick-save / quick-load while switching into Mercantilism to see how much Trade Route

Commerce

you'll lose.
2. If you're Philosophical or in a Golden Age, the Great Person Points

from the free Specialist can be very lucrative.
3. When you're in Mercantilism, you actually end up
hurting your rival civilizations by cutting off some of
their Foreign Trade Routes

. If you have a very large empire and there are a lot of smaller civs that you don't like, then it can be a good idea to switch into Mercantilism just to hurt them. The bigger you are, the less you benefit from Foreign

, and the more the smaller civs start relying on your

. So while it may hurt you a little, it may hurt your rivals much more. And the more it hurts them, the better it is for you!

4. You still get Foreign

with Vassal states, even under Mercantilism. So if you have a bunch of Vassal states, Mercantilism again can be an easy choice.
5. If you're on a Continents type map and you don't have Astronomy yet, then you oftentimes don't get very good

anyway.
6. Lots of the time, many other Civs are running Mercantilism, so you're not getting much Foreign

, if any. In those cases, it's often best to follow suit.
So in conclusion, the factors favoring Mercantilism are: other civs running Mercantilism, being in a Golden Age, Philosophical leader, the Pyramids, Continents maps before Astronomy, having a large empire, having lots of vassal states. I've never used it for its defensive role against foreign Corporations.
Environmentalism is usually a garbage Civic. If you're Spiritual, Environmentalism might be useful for brief moments if you have lots of

while building up Factories but before you have founded Corporations or have researched State Property. Other than that, it's generally only powerful if you're a small civ on a very

-poor hilly terrain with tons of Windmills, where you can benefit from the extra

... but
only if you don't need the

from State Property
and if you don't have Corporations, or at least not many of them. But if all you could muster is a small hilly civ into the modern era... that usually means that you've already lost the game. The difference between -25% Corporation savings with Free Market and +25% Corporation
penalty with Environmentalism means that Corporations are essentially out of the question with Environmentalism. And since Corporations are so good, that means that Environmentalism is pretty crappy.
The mod AdvCiv makes Environmentalism a valuable Civic in some situations, by getting rid of the -25% Corporation penalty and also by completely revamping the Global Warming mechanic, with Environmentalism being an important way to combat it. You could also conceivably use the extra

to sell away all of your

Resources for

/turn.
Emancipation: if you're the only civ left in Slavery/Caste System, and you're running the

to combat huge

, then you might benefit from the extra

by being able to turn the

slider off. The boost to Cottage growth could be helpful to Spiritual leaders maybe.
I used to over-value
Bureaucracy and avoid
Nationalism, but if you're not Organized, you can really save a lot of

by switching into Nationalism, since the latter has no cost at all. This is even before considering the ability to Draft units, which is incredibly powerful, especially drafting Riflemen from a Globe Theatre city. The extra

from Barracks can be very nice as well: and remember that you can sell your extra

resources for

/turn if you're not actively Drafting...but you probably should be. The

benefit from Nationalism is pretty niche.
Free Speech can easily be better for

than Bureaucracy, especially if you start conquering rich terrain with lots of Towns. This is even if you have a heavily Cottaged Capital with Oxford. Just do a quick-save/quick-load to see if it's worth it. This is completely ignoring the

benefit.
I still rarely use
Universal Suffrage, just because I think that Representation is so good. Maybe someone can show me how to properly use Universal Suffrage and the Kremlin, but until then, I usually end every game in Representation. I know that it's good with late-game wars with Corporations, but I don't usually like to get into those extremely drawn-out games.
I've only ever used
Police State in cases where I'm a Spiritual leader
and I have the Pyramids, during brief periods when I want to maximize military production. I don't think that I've ever once researched Fascism to be able to use Police State, since I'm usually happy enough to run the

slider a little to overcome War Weariness

, and because I value Representation so much.