I was browsing through WoWInsider and I came across The Queue. Mathew McCurley's short yet very common tale of a non-collector getting bitten by the bug is yet another example of what happens when Blizzard tacks on awesome pet rewards for achievements. Without any incentive, I'm pretty sure most non-pet collectors wouldn't bother with non-combat companions. It's not a good or a bad thing, it's just an observation.
Heck, even a few of my guildies who normally wouldn't really bat an eye at most pets have made it their own personal goal to reach a certain number for a certain pet. Particularly the newer ones (Nuts and the Celestial Dragon).
I can't speak for everyone else, but as an avid pet collector who would collect even if there were no rewards other than the pets themselves, I'm grateful to have even more friends and guildmates I can talk pet talk with. :P
Besides, even if they're only in it for a certain companion or end reward, once you start, it sticks with you. You'll be forever on the lookout for that next kick-ass, have-to-have pet. So they can claim they're not addicted and no, they're not "real" collectors... but time will reveal that it's not that easy to shake the pet collecting bug. :D