When there were 3 stable slots, I was much more careful with my pet collection since I was an actual raider. It pained me to be so. And then we got five slots. .... and two of them were permanently filled up with 'vanity pets'.
When the black warpstalker was first tameable, I picked her up (at level 72!), and she never left my stable, even when she wouldn't attack players. Even when she couldn't be buffed or healed. I joke nowadays that I sometimes regret they made her tameable, because she was such a special thing for me. But honestly, she is a beloved memory, rather than a super-valuable member of my lineup, and I keep her for those memories and the fact that I absolutely love how she looks.
On the other hand, my crocolisk-hydra is an extremely valuable member. He is my primary PVP pet. He's loud, intimidating, and a lot of people don't know what the hell he is or what he can do. .... that said, if hydras become tameable, and he becomes a hydra (as he is likely to do if they do), I may very well return him to the wilds of Sholazar because there are *SO* many other colors that make me go 'ooo'.