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.
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Yes, it would be absolutely horrible if Dropbox lost their non-paying users.
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.
Why would you need the other party to sign up? Isn't that the point of public links?
Also, if you need to start paying for whatever reason, these are the guys most of those users will go with.