I didn't downmod you, I think it's a good question. I think forcing users who won't pay to leave might be a good idea in some cases. Consider the cost in server resources (if it's a server-based app), customer support, etc. It may be too expensive to keep users who won't pay.

Of course, that assumes you can get plenty of paying users in the first place...

I didn't mod the previous comment but in answer to your question. Their are a lot of good reasons why you would want to for a user or set of users to leave your service. If for example you are not generating revenue from them and they don't appear as though they would ever pay then having them use a subset of your features would cost you money with little return.

There is the whole idea that they will recommend their friends who will in turn pay but the odds are if they are happy using the small portion of features you offer for free then so will the friends they recommend primarily because they can only recommend it based on the merits of said free features.

Not all customers are profitable and dropping the ones that aren't is never bad business.