Just something I've become accustomed too, extra damage and utility off the GCD that I can also use while CCed, ability to focus damage where it's important, beyond just splitting with multidotting. Extra damage during phase changes/downtime/outranging... There are an awful lot of things I miss when I'm playing SPriest, Mage, or even Aff with Sacrifice when I have no pet around.I do thoroughly enjoy my warlock and having a pet wouldn't ruin it for me, but it would give me a serious quality of life problem that I had happily forgotten about, especially since I have very limited methods of keeping such a pet alive.
Here's why I'm still a warlock:
Demonic Circle: Teleport
Interesting specs and class design
Purebred dps, thereby topping the meters by design
Why do you like having a pet?
Survivability isn't a problem, certainly not since avoidance came in in Wrath; there are some mechanics on certain encounters that can cause issues, but they're few and far between and more often than not are actually bugs that end up getting fixed anyway.