I don't think it's a coincidence that Dropbox took off just as smartphones became popular. People use a service like Dropbox because they're juggling more than one device. Dropbox has the advantage of being available on every platform and integrated into a ton of apps. Even if they weren't awesome on their own, Dropbox is the only service that makes sense now because none of their competitors has anywhere near the same network effects. Apps are their best marketing now.