I think pets are unfair in colo. On my Wiz I pretty much win if I get 1st place in the last 30 seconds-- 20 seconds of Executer Undead stunning everyone and then 10 seconds of ice wall.
On my rogue it's even more broken because I can use Executer Undead from stealth and then steal a kill while everyone's still stunned (so they can't dot me even if they wanted to) and safely smoke bomb again.
For reference, I just won the most recent colo on both chars.
Personally I think allowing pets is unfair because it doesn't give everyone an equal chance. Those who don't have a good utility pet that stuns, heals, dots, etc-- they are at a complete disadvantage. Sure, they could still win, just like people can win at lower levels and without VIP-- but that doesn't make it fair.
Someone in street fighter "could win" even if they start at a 1% HP handicap vs someone that has 100% hp...but just because they COULD win doesn't mean it's a fair fight. I know it's not the same kind of extreme, but my point is that colo should be balanced-- everyone should have access to the same things. IF they gave everyone access to the same pets, and allowed them to choose one for that colosseum... then I wouldn't mind. At least then everyone would have the opportunity to use the same cheesy pet.
I wouldn't insult a player or tell him "he always loses" just because he has a legitimate complaint about something that further breaks the scarce amount of balance we have in the colosseum.