Financially, it makes sense for the RIAA to target major traders, but the move should also help the recording industry in the PR department. It's still just as wrong to only be sharing a handful of songs over a P2P network, but I don't think anyone wants to see traders dragged into court over just a couple of songs.
Only targeting major traders is a risky move for the RIAA. With users knowing that sharing only a "substantial" amount of copyrighted material will attract the RIAA's eye, more people could flock to P2P networks for low-volume trading. If enough new low-volume traders are attracted to P2P networks, the RIAA could be faced with even more substantial copyright violations.