Good question, I'm running bitcoin client for several months, but i'm not sure i'll run it if it's getting too big to slow down my computer too much.
The computers are getting more powerful faster than Bitcoin is getting resource demanding. The opposite is true for Microsoft Windows. So any recent hardware are capable of supporting Bitcoin client. Tested Bitcoin even on aging server running dual Pentium 3 with Win2k3 R2.
It would certainly be pretty nice if there was a torrent network where seeders are paid. It would be like a poor man's Usenet but with the advantages of being decentralized and pay-as-you-go. Same thing for tor relays.
Some people prefer to stay anonymous, so only payment they might accept are Bitcoins. I actually run few Tor exit nodes 24/7, will You pay me for doing so? Who will pay? I will run them anyway even if You don't pay me. If You are short in cash, should You be denied from using Tor or torrents? Or there actually are people who like when computers and servers are idling to do some service for fellow internet users?
Only problem I have with running the client to support the network is it gives your ip away to would be attackers
Obtaining IP is the first and easiest step. IP adress alone will do nothing, there must be some vulnerability and way to launch exploit on the system with that IP or all attacks will fail and DDoS are the only option.