I think it depends on your starting point.
As a relative beginner you normally need to play a lot to improve. While only playing against stonger opponents is best for improving in theory, you also risk losing motivation due to the expected long string of relatively poor results (few beginners get the luxury of a strong enough partner to win in a tough field). Playing at least some of your bridge in weak fields or in more social games is probably good for you. You get to play a lot of boards, and will quickly be motivated by the fact that you beat more and more other players and occasionally get good results.
The situation is very different if you after playing a lot has become an expert, maybe with ambitions to improve into a world-class player (to use the BBO definitions). At this level improvement is much harder, and I actually think playing a significant part of your bridge in poor/social games may make you worse. In my own experience you both learn some bad habits and more importantly lose focus (you don't feel the need to concentrate to do well).
I know several very strong players who no longer play much at all, to me it seems that a break from bridge surprisingly often does not worsen their play significantly. I mention this since several posters have written that the alternative of no bridge-playing activities is much worse. In general I agree, but on the other hand a break can be good for motivation. When these players return they also seem to focus a lot, generally needing more time (things no longer comes naturally), which actually may be a good thing, since focusing is important for learning and improving. Maybe the best example is Ulf Tundal, who returned to bridge after an almost ten years long break, practically not playing at all (focusing on job and family). Granted he was a very strong player before his break, and he worked very hard particularly on system when he returned a couple of years ago, but last summer he won the Bermuda Bowl