Said by the guy raging about revolution ^^
In my 2nd biggest city, I have +4300 science, only 92 from Trade Route (with 100% science).
This city have ALL building about commerce and trade route he can build, with good civics about trade route. Even a wonder that give +25% from trade route. There a a river, so I can have the bonus from bridge building.
I have no idea how I can have more from tade route, and it's only 2% of my science on this city. For smaller city, it's even worst.
And I don't know how YOU do it.
Like Joe states it's all about different playstyles.
If I have 4300

I easily have more than 92

from trade routes in that city.
It's all about playing style and what one goes after.
It might also be that you are missing the bonuses given too? Commerce and science percentage increases, those count on your trade routes too, just not broken down there, but in your science hover window in a city.
With 25 cities you can have 24 domestic Trade Routes.
Granted this is with having all the Insurance/Trade technologies, roads, bridges, gatehouses, and all that stuff that increases # of.
This is not counting Foreign Trade Routes, which are worth more, only domestic.
25 cities, IF that is ones playstyle, remember it's all about playstyle, is relatively easy to get and maintain.
But, let us say 17 cities with 11-12 Trade Routes each instead. All Domestic. As that is the save I could find.
Each of these 17*11 routes give an average of 2.8

each, or 17*11*2.8=523,6

. (my F2 tells me 813, F1 Trade income tells me 853, but lets use the 523,6)
Given my playstyle and building a bunch of +

buildings I check a middling city of mine and find when mousing over the rundown on my

:

bonuses: +30% from buildings and +7% from Civics for a bonus of 37%, making the 523,6 tally up at 717,3

.
On top of that I find, in the same place:
+105% from buildings
+10% x2 from Scientific and Financial Traits
+40% from Civics.
Total of 165% increase, making the 717,3

tally up to 1901

.
This is of course a median. Some cities have higher, some have lower, outputs of

and

percentage boosts, but it serves to make a point (That PlayStyle is Key, and knowing how to read what's what perhaps).
My total science in that game is 7361/turn.
So, depending on playstyle, and how late in the game (earlier games with less percentage boosts and less trade routes at all of course give less of a bonus, on the other hand later games have these numbers increased a way bit more too) Trade Routes can be very effective.
I'll even supply the game save so you can check the numbers out yourself. Just remember that you have to know where to read the bonuses to

and

, they aren't plastered in any easy-to-find place.
Cheers
Edit: Use this as whatever proof you like. For or against Trade Routes. For me it's just how you use them and used right, maybe slightly exploited, they CAN be very powerful, but do not have to be. In the long run your Trade or Science may be off the charts but you'd still lose out to someone with twice your production rate and behind you in Technology War Monger type if you can't defend yourself.
Playstyle!