Also, since commerce gained by other civs costs you nothing, of what importance is it ?
it's situational, but sometimes it costs you
big time, since it speeds them tremendously to have your trade routes. even tho losing trade routes with them slows down your tech rate, the game's not a race as far as the calendar until you approach 2050. what matters on tech speed is how fast you are
relative to them. so if merc slows them down more than it does you, you win overall, even tho you lose some commerce. okay, your score at the end is higher if you win early so if you want a high score, then the calendar date does matter *giggle*.
i have city visibility for all my opponents atm, and cristo so i can swap civics easily. i did a test to check going out of merc, to lower my corp costs. the effect was insane! because my best cities are
so much bigger than theirs (i'm the only one with most late game health techs, and i adore big cities) and my cities have more base commerce (i'm not locked into automated worker logic like they are), they benefit more from the trade routes they get from me than i do from the trade routes i'd get from them.
gilgamesh's capital with me in merc, so he's limited to trade routes with other people:
gilgamesh's capital with me in FM, so that he has access to my juicy cities to trade with:
in this one city, the trade route portion of his base commerce before multipliers doubled. multiply that effect across all of his cities. i don't know whether they all doubled, i didn't want to check them all, he has a bunch, but we can assume that at least some of his other cities benefited at least a bit. with his slider set exactly the same in both cases (since i haven't hit enter), the amount of gold he gets per turn has increased by an order of magnitude. he's now making per turn as much as he has total in his treasury!
the turns til his next tech dropped by 3 instantly, someone else's dropped from 15 to 10. and of course they'll adjust their sliders when i hit enter, to increase science since they're comfortable making far less gold/turn than that.
meanwhile the gain to me was basically the extra trade route i got from free market. in my best cities half or more of the trade routes were still with my other best cities and not with theirs
. that's partly due to the map. it's hemispheres and i'd say 90% of my cities are coastal and whole bunches of them count as "overseas" to each other. then again, all of their cities are overseas as well.
it's situational. in this game my cities are lovely compared to theirs because of the health tech advantage and because nobody's ever declared war on me so i've had all the time in the world to make them pretty. that's not always gonna happen. i'm running rep and a lot of specialists thanks to sid's sushi, so the majority of my science isn't coming from commerce anyway, and the free specialist is really spiffy. even in this game merc wouldn't always have been a win for me. when i got astronomy, wowsa did my commerce benefit huge from their trade routes. back then it was 100% a win to let them in, even with the benefit to them as a factor. but in this case, at this point in the game, i see this as evidence that giving them access to my cities for trade routes is a cost to me because it speeds them up FAR more than it speeds me up.
i used to bring this up on the boards when folks talked about merc and some folks never would believe me. now with espionage, if you get visibility into their cities, you'll be able to see it for yourself. try it out when your cities are nice. save it first, since being in FM or SP might be a win for you
. but in some situations seeing the impact as it happens might make you think twice about letting them in.
i'm trying to take a picture of a juicy wall street city to show you, probably more detail than you need but i'm in a typing mood, so this is "to be continued" *giggle*
from buildings. that's the nice shrine and very powerful corporations, and i got some random event that added 1g to my market permanently (i didn't complain about that). 
