I recently decided to stop "serious" raiding because I noticed alts popping up in our raids. Coincidentally, Greedy Goblin posted about the No-Alt rule in his guild.
He explained why he thinks people play alts. I don't know if I completely agree with him on all points (alts used to grow epeen), but there is truth in it. If you are on an alt you are there to soak up gear, rep or emblems not help the raid progress. I've blogged before about why I think alts are a detriment to progression raiding. At the point our raid started allowing alts I knew our goal had changed from progression to something else.
9 hours ago