From Dropbox's perspective, this sounds great. Accounts become more useful and valuable. The addressable market of a Dropbox account grows! Plus, everyone has a Dropbox account already, right?
Unfortunately, it turns out that business customers generally don't deploy Dropbox wall-to-wall. It's expensive. Not all employees need file sync.
A Dropbox account ends up being an obstacle to adoption.
And a distraction: a common account creates an irresistible urge to spend a lot of time finding ways to tie this new product into the old one.