I know its not a great idea only because all someone has to do is dump their internet cache in the folder, but I'm talking about is not just number of files but also the amount or size of them, because you have people just dumping garbage on the network.
And yes, even if you are a modem user, you should share! Limewire and I imagine some of the other gnutella clients have the ability to trim the upload bandwidth being used. And even if you don't share files, the client takes alot of bandwidth anyway!
So if you are a modem user, you shouldn't freeload but even more to the point don't share crap. Share things that are worth something and simply tweak the settings of the program to your connection(don't accept as many out/in going connections and trim the upload bandwidth off you).