[BluffTHIS!]

If you allow yourself to play bad against bad players by giving them too much action in certain situations where they are ahead, then you have given up a large part of your edge against them and also given them a freeroll to play more hands against you and suck out.

Although the option you mention, calling and then folding if the flush comes is viable in some situations, you are also setting yourself up to be bluffed off a split by another player overplaying a nonfreerolling straight on the turn, but who is capable of reading your hand and bluffing. Whereas you can't do the same to him with any confidence of having fold equity because he is more likely to be a calling station. So wait until your hand and your position and the opponents involved add up to a better spot to stick your stack in.