For raiding it's generally best to have the pet on passive and manually select the target to attack. Not just for a fight like Blackhorn where the pet can't reach certain targets, but any AOE fight as well, because the pets spend so much time ping-ponging to different targets. Also I think as a pet class it's part of our job to manage our pets and not rely on the AI.
But, I do miss the old version of assist stance as well where the pet AI was a bit more intelligent.
Generally I agree with you, but using a /petattack and a /petpassive macro generally fixes that, while on trash it's just plain easier to keep it on assist and not have to worry about sending it in (and forget)... Generally, I'd say that the right thing to do is to switch between stances on bosses/trash, but well, I forget. *shrug* What's funny is that in the old system, I actually kept my pet on passive. >_<
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