I don't want to sound like a dick but aren't you admitting I am right?
Yes, you are getting dropbox credits by performing marketing for them, in whichever way is good for you. This might be advantageous to you but it is wrong to call it "free", at best you are paying with some kind of barter.
You're right about one thing: pay for things that you're actually using. That's what modern economy is all about and that's why we had (still have?) a recession.
I don't use or need more than 4GB of Dropbox space. I found promoting a service that I use daily good for me, people that follow me and good for Dropbox because they're getting more quality users from me than from other sources.
The thing you're wrong about is that there isn't really any difference for Dropbox if they offer 20GB, 10GB or 100KB for free. They are still getting payed from people and corporations that use way more than us. That free quantity they offer is negligible.
Yes, you are getting dropbox credits by performing marketing for them, in whichever way is good for you. This might be advantageous to you but it is wrong to call it "free", at best you are paying with some kind of barter.