Commerce has and always will be my favourite tree - ever since vanilla
The purpose of Commerce is to give your gold more purchase power by reducing the strain on your GPT, as well as generate more with tiles (Trade Post/Customs House/Opener); this essnetially makes GPT a secondary source of production. Gold is more versitile than hammers, as you can: spend it to get happiness, spend it to get culture, spend it to get faith, spend it to get food, spend it to get units, spend it to maintain big armies, etc etc. Production can only ever use 25% of its output on science, or gold - and everything else production can do relies heavily on how much GPT you're able to generate.
A few points in defense of commerce:
1) Reduced Maintenance - boosted from Vanilla & G+K from 33% -> 50%, this makes it so you spend 0.5 gpt on a road, and 1gpt for railroad - This bonus helps make city connections more profitable by reducing the strain on your gold, as well as makes the 25% production bonus from railroads be more cost efficient. It's not the biggest bonus for GPT sure, but the added benefit is that its possible to build redundant road systems at a fraction of the cost in order to help maneuver your troops through rough terrain.
2) +1gpt Trade posts - Anyone who goes around puppeting the world shouldn't complain about this boost. Commerce really is a warmongers best friend when you can generate an extra 50% (after economics) off of trade posts. Wide puppet empires make tons of money with this finisher - and the extra GPT means more buildings/units can be supported - as well as more gold for the power of rush buy.
3) 25% reduced rush buy cost (15% from Big Ben Commerce exclusive) - the most powerful skill in my opinion. This policy is what turns your gold from what you use to maintain everything, to production; getting Big Ben is icing on the top. Granted, if you've got an empire thats producing <100-150gpt than this wont seem like a strong Policy - but for those who are abusing trade routes, trade posts, and customs houses and your GPT is well over 150 then sit back and start buying everything. Most of my own games I'm producing around 300-500gpt - this translates into a 1 Factory every 2-3 turns; or 1 combat unit ever 3-5 turns ON TOP of the production i'm already getting from hammers. This benefit is further boosted with Autocracy by reducing rush buy on units by an additional 33 % - thats a 58% (73% w/ BB) on buying units. If you have a city that happens to have Brandenburg Gate, then you can rush out so many Elite units at a really marginal cost with gold - FASTER- then you can probably build with hammers.
4) Synergy - Commerce works very well with OTHER SP and i think this is the most important part of Commerce.
-Patronage - Commerce benefit from one another with lots of GPT to buyout CS, and lots of Happiness from all the luxuries. (I believe the bonus would be 8 Happiness from CS luxuries with both happiness boosts from Commerce and Patronage)
-Honor - Commerce - Autocracy fields massive elite armies at the fraction of the cost (Reduced Unit maintenance, Reduced upgrade cost, Reduced Buyout)
-Commerce - Rationalism - Freedom get great use out of trade posts/GP Tiles and Great Person generation, the Aesthetics opener also helps