South Boston isn't appropriate for a football stadium. Yes, it'd get used by the Patriots' home games (plus applicable playoff games). Sure, there'd be concerts throughout the year. Indeed, if a stadium were built in the urban core, the Revolution probably would not get the soccer-specific stadium they need. That gives a grand total of about 30 uses a year (maybe 40). Given that the Revs only take in on average 10-15,000 a game; and given that with few exceptions, most football stadia require massive amounts of parking (as someone stated in another thread, cars and football are more closely associated than any other sport), a stadium would create a huge dead zone. The possibility of getting the Super Bowl once, maybe twice, in the next fifty to seventy-five years (maybe) doesn't justify that.
The Revolution need a soccer-specific stadium.
A soccer-specific stadium with a capacity of 20,000 would allow for a more intimate setting. Plus, a spot that is transit accessible will enable more ardent soccer fans (like the Brazilians in Cambridge and Somerville) to attend games. A smaller stadium would also allow for more energy. Even with 15,000 people, that means there's another 53,000 seats at Gilette sitting there, empty. Even if one recognizes that the Revs could never fill Gilette Stadium, it takes a psychological toll seeing all those empty seats and it drowns out a lot of the cheering.
The football stadium is already built. No point in crying over spilled milk. If you want to talk about a stadium, let's talk about the aforementioned Revs stadium. That has a fairly good shot at being built and it makes sense.