This is a very clever move by Dropbox.
My outsider view: they are going to build a full-fledged productivity suite. They have the storage, versioning, you can view files/docs and now you have an email client. The "obvious" next move would be getting into the online document editing space. But then, this is me just speculating from the sidelines.
Meanwhile, the mobile e-mail client that the Mailbox team built is so cool that it convinced about a million people to let Mailbox store a full copy of their e-mail archive ... in 10 weeks.
That kind of strategic value means it's smart for them to swoop the product and team up RIGHT NOW, even if they had to pay a premium to do it.