Um.. yeah. In case you hadn't noticed there's a threat bar by each player's name.
Having your tank pet out does pose problems for another tank. Now yes, a good tank can still keep aggro. But why are you as a Summoner lowering your dps for having a tank pet out when you already have a functional tank? (granted if you don't have a functional tank, or a terrible one, then that might be cause to have your tank pet out.. but otherwise.. no)