You must be thinking that I’m going nuts (I wonder that myself!) by writing this. Let me assure you, this is not a conscious decision, just the observation of a fact. More and more, when writing papers with external collaborators, we end up sharing the paper using DropBox instead of setting a SVN repository for the paper.
I guess the two main reasons are simplicity (no need to create and manage user accounts for the external researchers) and extreme laziness (DropBox synchronizes the files automatically but with SVN you need to ask for the update yourself).
For sure, the merging algorithm in DropBox could be improved so you better avoid working on the same parts of the paper at the same time (already happened to me once that while working on a file, DropBox “synchronized me” and lost the changes; I learnt the hard way to pay more attention to DropBox warnings!).
Are you brave enough to confess that you are doing the same? Come out of the closet!