I can think of quite a few reasons why someone might want to run p2p software on a tablet:

1. You don't have a high speed Internet connection at your home and have to go to an internet cafe, workplace or library to download large files.

2. You don't want p2p activities traced back to your home computer due to the suit-happy RIAA, so you do your downloading elsewhere.

The N800 allows for some very hefty SDHC card sizes, so you aren't limited to small files either, necessarily. Battery life would likely be the limiting factor.