I have the feeling this is less about rares that already share a skin with normal mobs, and more about the rares that have a unique skin, a la King Krush, Araga, OCJ, etc.
My feelings have two parts.
Part one: I tamed Snarler because he was rare. I know I could have tamed a Vilebranch Wolf (think that's the name, anyway) much easier, but I wanted the rare. I didn't care that it was the same skin as the other wolf, and I know no one will know the difference. But I don't care! I love my little Skaza because he was what I call a "particular tame". I like rares because I feel like I am hunting or in search of a very specific creature, not just any ol' wolf wandering around. Rak'shiri was the same thing. I could have saved a lot of time by just summoning Shy Rotam, but I didn't want that. I wanted Rak'shiri, dangit! Even though no one can tell the difference, that doesn't matter to me. She's the one I wanted. And when I look at petopia for my next pet, whether or not that skin has a rare or unique mob plays a large part in my decision. Even if I like a particular skin, if it doesn't have a named or unique mob, chances are I won't get it. I'll find something else. It's not because of what other people think, it's because that's the part of being a hunter I enjoy, and I'm very particular about my pets.
Part two: I tamed King Krush because he was rare. There are no other green devilsaur skins out there right now. When someone sees Apollyon, they know exactly who he is and what I had to go through to get him. I love him just as much (actually more than Snarler and Rak'shiri), but he has the added bonus of being a recognizeable figure. He's the one and only, no other devilsaur like him. He's available to everyone, depending on your wallet and your luck. Some people need more time to find him, some people less. There's ganking, griefing, etc. That's why it's such an achievement to have him. I feel proud to have him. Not the kind of ego-pride, but the pride of seeing my beloved green dino stomping happily by my side. I pull him out just to look at him and /cuddle him. I cherish him both because he's rare and unique, and also because I lost literal, RL sleep over him. I dreamed about him, and then that dream became reality. So yes, yes I would be a little upset if suddenly, my most prized pet was made common.
I know what I went through, and I'll never stop loving him. But the other part of what makes him so special, that individuality, would be gone. He wouldn't have people /clapping at him or fondly reminiscing of how he pwned them while they were questing in SB. He'd be just another devilsaur. The same goes for Araga or OCJ. They may not make you want to throw yourself off a cliff when you're trying to get them, but they're unique. People know them when they see them, and it is a source of pride for the hunter. Again, not ego pride, but the kind of pride you feel when your child wins a contest or something. Like..."yay!" pride.
Now, if it came down to either retiring King Krush or making him common, I'd still choose common. I don't want the joy I feel to have him to not be available to whoever wants him. But honestly, I don't want his skin becoming common either. Before I saw King Krush in person, I said that it was a shame there were no other green devilsaur skins. But as soon as I saw him there, and then lost him...I didn't care if there were thirty other devilsaur skins like that. I wanted HIM.
And oddly enough, I want a red-brown devilsaur skin in Cataclysm. Who do you think I'm going to tame? I'll give you one guess. That's right, King Mosh. I will put myself through that torture and aggravation just to have what I feel is King Krush's little brother (especially since Un'Goro and Sholazar are connected, how cool is that?!). Even though no one will know it's Mosh, I'll know it's Mosh, and I'll love him, too. But it's even cooler that his big brother Krush is the only one of his kind.
Am I a selfish, greedy a-hole? I like to think I'm not, but that's up to you guys to decide.