I assume you're being sarcastic. Non-paying customers do server at least one purpose for Dropbox, word of mouth advertising and the fact they tell others "I'll just share the file you need on dropbox. Go sign up, it's free". All these non-payers, and the people they refer, may start paying at anytime in the future for whatever reason.
Also, if you need to start paying for whatever reason, these are the guys most of those users will go with.
> I'll just share the file you need on dropbox. Go sign up, it's free
Why would you need the other party to sign up? Isn't that the point of public links?