Please.....P2P isn’t half the problem that rampant spammers are.
I haven't read anything that analyzes the bandwidth impact of spam. I wouldn't think that it's all that bad, overall.
All email combined isn't a large percentage of the Internet's traffic. So even if spam is 80% (wild-guess percentage) of all email traffic, it still wouldn't have a huge impact.
P2P is a major bandwidth hog, and it's a long-term one, with hours and days devoted to huge media transfers. EMail is delivered in relatively short bursts.