The overhead of maintaining separate Dropbox accounts is pretty high IMO. There's no easy way to switch between accounts on the desktop client, an I'm pretty sure there's no way to be logged in to multiple accounts.
EDIT: Found the link: http://maketecheasier.com/run-multiple-dropbox-accounts-in-m...
Another thought is that on personal Google account I only keep couple TrueCrypt containers, based on size and open read/write close them any time I need the access. This is necessary because once you unount TC container, entire file is affected so it needs to be shipped back to the cloud. I have couple 50MB ones, some 100MB and some 1GB ones. I know its not the convienent way to access your files, but I dont want Google (or drop box) to have open access to my files.
Of course, there's also an opportunity here for Dropbox or a competitor to offer something better. I expect there's a non-negligible demand for the ability to sync multiple accounts to different directory sets on a single client. So, maybe it's a feature that could be offered only to paying customers, to allow them to more easily separate their personal and business accounts.