Commerce is worth it just for the ability to pull supplies from the market AND the stockpile, almost doubling what you can access in one visit.
The tarrif is so ignorable I didn't even realise there was one until it was pointed out.
Using a building slot for commerce is only a problem if you try to build a colony that does everything. This is less of an issue once you start moving toward specialised colonies that support each other.
Question:
Interaction with a market is one of the most basic mechanisms in this game, so why is that gated behind something just because a place is owned by the player?
Double dipping may be handy early, but once colony gets big, stockpiles produce more than enough. Burning through income would be more of a problem than running out of stockpiles. (Emphasis "would" because player probably still has plenty of income to spare if his colony is big and exports everything.)
Game breakers aside, building slot for commerce is the biggest problem now. It is one of the first to get shoved aside as I run out of building slots (provided I do not abuse the exploits).
I try to pick planet that can have everything, allowing for completely self-sufficient colony. That means Commerce, Tech-Mining, and Planetary Shield get shoved aside.
With industry limits coming out, player might be forced into specialist colonies. I guess specialist colonies are handy for evading active pather cells (although that means no synchrotron for fuel plant).
P.S. If Open Market is not standard, then I probably would not want Commerce in my important colonies (where I store important stuff) because inconsistent defaults means muscle memory or whatever can hurt me if I do not pay enough attention. For example, take or drop stuff where I think is Resources when it is Open Market. At least ships have a confirmation dialog before buying or selling, and that saved me from taking or dropping a ship into what I thought was my storage into Open Market instead. There is no such safeguard for items since the UI is identical. Free Open Market for all colonies could at least enforce the same default market when managing colony.