If I store a program on Dropbox, I might run it many times. Thousands? Millions to be generous? But if the attacker has to do a brute force search which relies on me executing each different tampered version of the program, that's probably too small of a number.

The reason to accept sub-optimal properties in this case is convenience. Truecrypt is popular and has a nice interface. Using it with Dropbox gives much better security than not using it with Dropbox. It's not perfect, but I still don't think we have any practical attacks against such a scenario.

I agree that having stronger crypto there is however possible and desirable.