Here's how I think about it.
While I am exploiting the system, dropbox really don't care that much. With services like box.net now offering 50GB (albeit it is a somewhat different service), it would be stupid of dropbox to alienate users for any reason (like zealously hunting people who just want more of their service). They are most definitely the best service right now, but all that could change tomorrow, or a week from now - all their efforts are put into making sure it stays a good service. There are a ton of rather simple and easy ways to make it much harder to trick the system - but they really just want the best user experience possible, so as an example, adding captcha's to their signup pages, to defer people like me, simply is not an option. I applaud them for being so focused on the important things.
My point is, the last thing they want to do is fight their users. Even those who bend the rules.
Everything these days has to be black and white. I refuse to look at the world that way.
I'm super psyched that I'm able to improve the experience of an already awesome service. That's part of the reason I don't charge a fixed amount. I'm not here to rip anyone off. The main mission is to get everyone that extra space they deserve ;-) I would probably do this for free given no other option, but on the other hand, it's a way for people to show a little appreciation, as well as a huge motivational booster for me.
In the end, I do understand why some might frown slightly, weight the risks, and opt-out.
Well, this is kind of black and white. Sorry moredropbox, as much as you are a super-nice person, we all know that this is not the way Dropbox meant. They sell storage for money, and they "buy" customers and pay in storage. That is the way we should look at this. If we sell them a customer (refer a friend) they reward us with storage, just like any other transaction.
And what we are doing is (a very small and insignificant) fraud where we pay moredropbox to sell Dropbox made up customers. Conclution: it's not really right, but what the heck, it's really nice!
Also, you don't wanna ruin your day but there are actually people who trade very much more bad things with bitcoins. There are people who trade dangerous drugs and weapons, so, we should argue with them instead of this.