> There is a case to be made that charging less than $10/month for a service is not good business.
I don't think so. Just last week I signed up for unblockus.com; $5/month. Every domain name registrar on the planet - $12 a year or so. Full automated services should not be subject to a monthly minimum, and they shouldn't need support.
> people who pay low amounts are the most demanding in terms of support
Do you really think this would be a big factor for Dropbox? If so - just say any under-100GB account has the same support as the free account.
> Is $10/month too much for you?
Well no it's not, obviously, since that's what I pay. But I am the only person I know with a pro account, since it's such overkill, and even I kind of resent paying for so much more than I need.
I like Dropbox, I want to use it. Just let me pay an appropriate amount for an appropriate usage!
> you don't need to spam friends, there are regularly things like contests (Dropquest 2011, 2012) where you get free space for completing tasks
I am guessing you are pretty young. You will be amazed at how quickly your tolerance for such things decreases with age. Free space for completing tasks? That does not even make sense, because if you have to complete some task it is obviously not free. How about I just do my job, and buy things I need (like dropbox), and everyone is happy.