In her comment on my previous post about cliches, Susan has hit the nail on the head (there's a good one!) in her assessment of why they are so useful: they vividly combine truth with brevity. But therein lies the danger: because of their popularity, they become so familiar that we can fall into the trap (hey, there's another!) of resorting to them without thinking about whether they say what we really mean. Then they just become filler for lazy speakers and writers. Worse, if we're really being neglectful, we can end up mixing our cliches so that suddenly we're making hay while we smell the roses or some such silliness as that. George Orwell wrote about the pitfall of letting words guide thought instead of vice versa in his great classic essay "Politics and the English Language." And that's what I am getting at when I caution writing students about cliches.
But I agree--sometimes they can't be beat for getting at the meat (I'm on a roll today, aren't I?) of what you want to say.