I'd rarely bet an inside straight draw, because the chance of hitting it is too low. An outside draw is better, and on the flop, you'll hit your draw roughly 1/3 of the time, and a flush draw has slightly better odds.
Unless you're heads up, the odds can then work out at least around break-even before you add in the 'fold equity' and value for disguising your hand and mixing up your plays.
Once you factor in the other elements, it can be a very worthwhile play. If it's heads up, then it may be an expensive mistake, but as mentioned by someone else, it can be useful as a semi-bluff - you're really hoping to pick up the pot there and then, but unlike a plain bluff, if they call then you still have a half-way reasonable chance of winning.
Also, bear in mind that in both situations pairing your cards may also give you the best hand, so your odds on improving to a winning hand may be better still.
All in all, in a cash game, it's often then worthwhile. In a tournament, the need to protect your stack can make it a less worthwhile strategy.