The more competitors Dropbox gets, the more fragmented the whole thing becomes, which in that sense helps Dropbox. I look at Cubby and think "not another cloud storage", there's no point for me even if it's "better" because I'd have to convince everyone I work with to use it. I'm already sharing various folders with other people on Dropbox, the switching cost is large and only GDrive looks like a true contender as something that many people will end up using or at least will bother to install if I need to share something with them.