Besides the obvious factor of mutual protection owning to a diverse skill set -- a superhero invulnerable to blows might be very glad of a superhero with wind powers against a foe with poison gas -- there's the little matter of legal protection because of the danger they attract.
If supervillains set out to prove themselves against superheroes, knowing a superhero is in residence would attract them to the city Heroes who let it be known may be liable for attracting them. Better to move outside the city line for your residence. Very likely many superheroes would have to pool their money to make it palatable. Even super-other-occupation might find it legally useful; a building inspector with X-ray vision might even find himself more targetted by villains who assume he's weak. (Mind you, it's not perfect, because you would still be known to work there, but it's better than nothing. Secret identities might also be a factor, if they are allowed.)
Not to mention that being thus concentrated would also help you with schools for your superpowered children, and other such amenities.