Current traction and paying customers may not be enough to satisfy their growth/evolution goals. The cash could be used for a lot of thing. One guess is that they may be trying to ramp up the attack the enterprise storage/backup market and need a war chest to have any major impact.

Also, plenty of people who could really use a service like Dropbox have not heard of it. Even with their current product offerings, there is an ocean of customers who haven't been reached yet.