I think it's pretty widely accepted that Dropbox is used for personal stuff a lot. I know at least a dozen people who use it, including people who don't work anywhere near tech.
In fact, remote storing of business data is a huge red flag for a lot of people, so many businesses do not use the service.
Filmmaking, and particularly indie filmmaking, is a very distributed operation, in terms of assigning work and often in geographical terms as well.
You may or may not have an office for the project, you may or may not have a base of operations. You'll have dozens of people who need to coordinate frequently (constantly, really), though. And Dropbox is a major help.
Scripts, schedules, budgets, art, paperwork (reams and reams of paperwork), call sheets, gear lists, location scout photos, maps, breakdowns of 100 different sorts, VFX tests, various collections of footage (though production footage tends to be large enough to warrant sneakernet), etc etc.
I sometimes do coodinator work of various sorts on low-budget movies, music videos, etc. Dropbox made very quick inroads into that world. Even if the production doesn't have one for the whole project (and they should), departments will often set up their own. Particularly art departments, with their 50-bajillion details to track.
I've got a couple of film projects I'm developing with partners, and we use Dropbox from very early in the process. Scripts, storyboards, concept art, etc. One of our partners is in Portland, and another in New Zealand, but Dropbox keeps us all on the same page (along with Skype and Gmail, of course).
In fact, remote storing of business data is a huge red flag for a lot of people, so many businesses do not use the service.