I don't think it's a question of what good is Perrin, but a question of when he will become a more significant force. As of right now, Rand is the only one that has made any significant accomplishments. Perrin so far as purged the Two Rivers of Trollocs, set the Two Rivers in a totally different direction than it was bound, and dealt a serious injury to Luc/Slayer. Incidentaly, he is doing a lot better in the dream world than a lot of the Aes Sedai. So, now he's on his way to deal with Masema and possibly return with an army of dragonsworn. And as stated in Shadow Rising by Thomas (or Ivon, can't remember which Warder it was) he has a natural eye for the battlefield. So far Mat is leading a fairly large army, dices a lot, and has just manages to stay even with the girls. Oh yeah, he killed Couladin. Unfortunately, that didn't slow down the Shaido Aiel very much.