I think going to lunch with coworkers (and bosses) is practically essential unless they're just really truly unbearably awful (and if they were you'd probably just resign?).
A coworker of mine was just talking today about how folks in my dept at work, while out having drinks, just decided (despite it being very much discouraged by the firm) to share details of their salaries with each other and now people know if they're getting shafted and taken for a ride salary-wise, and a lot of people seem to have bailed recently or are ready to bail imminently, which I am supposing is related. You might never get to know something like that if you don't cosy up to your colleagues a little bit.
Being in the loop with office gossip helps in progressing I think, it means you always know how to position yourself to your best advantage in relation to everything that's going on around you.