DropBox works great for me on OSX, Windows, and a couple of Linux variants. I just can't see Microsoft, Canonical or Apple going out of their way to make things work well across platforms.
And Google Drive probably ties into Google+, which kills it for me.
If say, Microsoft bought Dropbox, would you then no longer trust them?
Interested in the reasoning here.
Second, possibly related to the first point, it seems the other companies are doing cloud storage to sell their other products. SkyDrive ties in with Office Live, Google Drive with Google+, etc.
There's always a risk a company will either shut down a business (Symbian, Wave, MySpace, Delicious), or go out of business (Chumby), or get bought and have their business shutdown (Sidekick, Drop.io). I don't think you can really know the future enough to make product choices based on the chance that will happen.