When I used bnet to crack some rar password, after few days of work I noticed about 20% drop in quantity that never really recovered. And it was not worth the efforts, since the rar file contained basically crap.

Bandwidth, some bursts of activity - ok, this is not noticed by most losers. 100% CPU load might make machines too unresponsive to alert some seek for help. And if the loser is joining other networks as well, the computer in most cases is like Pentium 2 struggling with Windows Vista.