The nice thing (tm) with dropbox is that it is OS agnostic. I can have my files synchronized on all of my platforms - easy.
With me being on *nix/win/android/mac platforms (at work/uni/home) it really pays of that dropbox is not requiring me to stay on one
OS at all.
To make the case with WinZip, if the included zip extractor found in the operating systems where so that you could only open "windows zips" on windows, and so on.
Then WinZip would still be in great shape!