I can respond to the beta forums, but given what a cluster it is in there right now, I feel more comfortable with fellow pet collectors. XD
I like the idea of the 'shiny' system from Pokemon coming into play, where you might just catch that rare shiny that's a little bit more powerful than all the others - but I've been on the beta, and I've gotten several rare pets, and I've found that they are significantly stronger than the 'common' and even 'uncommon' variety. While I do feel that 'rares' should be in game and should be a little stronger than an average pet, the scaling should not be as extreme as it is now. People should feel like getting a rare is an extra little fillip to brighten their day - not a necessity to make it in the world of pet battles.
That said, I was reviewing the list of non-capture pets going rare and was bitterly disappointed to find that all of the 'no longer available' pets such as the Spirit of Competition (by far the prettiest dragonkin in my book) or the Vampire Battling are not considered rare - despite the fact that they are some of the rarest pets in the game (as they are unobtainable), and yet pets you can shell out real-world money for made the cut. Looking at that list of 'upgraded' pets tells me that Blizzard didn't really consider rarity or difficulty of obtaining when they made the list - they considered the advantage to them in money spent, time spent playing, or satisfied AH traders. -_-