Well, I don't think the Linux market is big enough to make it a compelling argument (but I really don't know; I could be dead wrong).
As for security, I'm not sure how big a problem Dropbox has with security. And I would't open-source a client just to get better security for it, but maybe that's just me.
Open sourcing the client is betting on the fact that the client doesn't matter. Maybe it doesn't matter, but maybe it does. More importantly, maybe we think it doesn't matter right now, but we'll find out that we were wrong in 2 years, after it's too late.