Not that Marc Bodnick doesn't have this spot on, but I wanted to add a couple of things:
1) Why didn't Dropbox acquire Mailbox before it launched? I mean, the hype was below the current peak right? So the price would've been lower! Plus, things would've been less hectic and more "settled", per the original question. This game of trying to time the buy is ridiculous because it contains assumptions and guesses upon assumptions and guesses. Who will grow faster? Who will be considered "hotter"? Who will suffer some black mark? Most acquisitions occur when the acquired company is at / near peak value because most acquirers only buy shiny things when they themselves are feeling sparkly... It's not optimal from a spending standpoint when you are paying cash, but when you are buying with stock it often doesn't matter.
2) Let's play a quick game of $100,000 Pyramid. "Unicorns"... "Imaginary Friends"... "Battles between toy soldiers".... Oooh, ooh... Made-up things! Kind of like the valuations of these two companies. On the one hand, you have privately-held Dropbox, last valued at ~$4 billion in an inefficient VC market based on a finger-in-the-air test and some multiple of revenue that's currently "ridiculous" and may well be vindicated in the long run. On the other hand you have Mailbox, a company with no revenues, no revenue plan, and completely uncertain prospects. You pay... $25 million? $50 million? $100 million?
Whatever the case, you are looking at <3% of the total value of Dropbox and no meaningful portion of its so-called runway in terms of its available cash. In fact, I'd be stunned to learned that Dropbox isn't cash-flow positive already. Is there any scenario where adding a couple percent in headcount and diluting the equity by a similar amount is going to tank Dropbox? No realistic one I can imagine.
If, on the other hand, this app revolutionizes e-mail -- to be determined -- it could lead to a halo effect for Dropbox, which has meaningful long-term implications for the company. In 6 months, it might already be known that it will do that, the price will be higher, other acquirers will swoop in with much bigger offers, Dropbox will have missed its chance. Today, it takes a small risk for whatever small chance of a large reward.
Given that risk profile, why exactly would you wait?