It does not "obviously" amount to theft. It *is* illicit, and it may be immoral (see Free Rider Problem), but it is not theft. If I steal 10 M&Ms from you, you have 10 fewer M&Ms - not the case if I download your song, in which case you have less than you otherwise would have *if and only if* I would otherwise have paid for it. This clearly is not the case for, say, college students with tens of thousands of dollars "worth" of media on their hard drive.
As for legal uses of "file-sharing" technologies, well - how about the entire world-wide web? We're sharing files...
Specifically P2P file-sharing technologies? Linux ISOs and WoW updates, to name two common legal uses.
Finally, I for one have an emotional reaction to assertions that technology should be restricted unless I can make you understand what it is for - and I don't even personally use any P2P software at the moment.