It really depends on what you think the EV is. Dropbox has pretty high EV if you're in his role, I bet, which is why people are speculating in the first place. (Does this mean Dropbox is not as valuable as it seems from the outside?)
Personally, knowing how hard people there have worked over the last 6 years, I imagine he just needs a break.
Yes, obviously $100k of Dropbox stock is worth much more than $300k of $randomStartupStock. Although at a VP/C level status, I'm not sure how much of a difference an extra few months (or even a year or two) of vesting makes.
If you have a sizable chunk of equity and the liquidity to exercise options (or if they are already exercised) then leaving the company can make it easier to trade on that equity.
I take it you're referring specifically to (1) lock up periods and (2) the negative signals you send when a CXO liquidates shares while still being a CXO?
In other tumultuous times, the dot com days, there were many examples of people simplifying all the rules about when to trade or what they could trade, by ending their employment. I did not take that route but many did.
I was remarking that when the future has many variables and you want to simplify, that can be sufficient motivation to leave a company.
He joined Dropbox after it became common to have ROFRs and other terms that made secondary sales less possible. I don't know the terms he has, but I'd be surprised if Dropbox stock is being moved the way, say, Twitter or Facebook stock was back in the day.
Personally, knowing how hard people there have worked over the last 6 years, I imagine he just needs a break.