As a product focused company trying to create a great UX, it's really quite easy to justify something like this for creating a better UX. I can imagine a PM loo

As a product focused company trying to create a great UX, it's really quite easy to justify something like this for creating a better UX. I can imagine a PM looking at the collaboration flow, UX researcher finding friction in initiating collaboration, testing mockups where Dropbox "magically" knows your colleagues, and it getting great reception from users. Most users still don't care about privacy, most users assume email addresses are essentially fully public. This part is a better UX for the majority of users. The problem comes when there's a data leak, or when the marketing team decide to email those contacts, or some time later that could really easily be argued as "this will never happen", and may often not, but it's possible. It's details like this that differentiate the "good" product companies from the "great" product companies, specifically because the detail of "not collecting email addresses" would never be noticed by those users, by construction.