First, if you instantly start another pet summon after the last cast, your pet won't get the "Summoning Disorientation" debuff. This is useful if you realize you forgot to do the voidwalker sacrifice trick just a few seconds before the arena starts.
Secondly, if you have a summoned pet, and you mount up before you enter a battleground, when you enter the pet will be active by your side. This is useful because it lets you keep soul link up while you are mounted, and if you have a felhunter out you can see rogues more easily. It generally makes getting the opener easier and better, and will reduce the damage you take if you get jumped.