Someone in another thread suggested that there should be a title/banner (or other prize) for leveling all of the pets to cap level. I think this would be an awsome idea. Get one vanity banner for leveling your character to cap, and get a matching helm & armor vanity (and/or title) for leveling all of the pets to cap. Obviously you would have to have all of the pets (except "extra" ones like Snowball & Ripmaw), and all of them would have to reach the cap level.

It would certainly take a long time, but would be worth it if the prize was right.