And to add to Icemonkey's reply, if you're playing at a shorter table the blinds come around more often so collecting the blinds keeps you afloat. On top of which if it's folded to you in late position, there are only a few hands still playing so even a marginal hand may be the best one out there.
You do, though, have to be able to fold when you meet resistance, especially from a tight player. Some players, once they raise, they can't let go even if it was a steal attempt.
And, there are those beautiful times when you raise with crap, get called, and flop an unexpected two pair, trips, or low straight.