This is exactly the logic that our mteam used in the '99 bubble when we built out offices on 2nd. I'm not saying Dropbox is extravagant, but this isn't a compelling argument to me.
Is Dropbox overspending on employee perks? I highly doubt it.
Does that make every expense that can be rationalized in comparison to headcount, revenue, or (worst of all) potential pull-through revenue valid? No! Some are valid, some aren't.
All I am saying is, this notion of "we need the very best talent, and we need to make it is as happy as can be" is a platitude that was used by execs to justify a lot of excess.