Gonna be long but some background is necessary as to why people use to flip out and probably still do when mages bring pets to raids.
Mage pets were used as main tanks during raids at one time. This pissed off actual tanks as their usefulness was basically very weak dps. The Sony dev's heard the complaints about how mage pets could hold aggro better than warriors so the mechanic was changed so mobs will attack any player over a pet.
So now all pets were relegated to dps and actual tanks were once again tanking mobs for raids. The holy trinity became one playstyle but a different style of playing also evolved; pet groups. As long as a pet group had some sort of healer the group would be very effective. My wife and I regularly played in these groups. Her first main was a druid but she also had a shaman and chanter. I mained a mage but also dabbled in necromancy. Most of our friends also played pet classes. All pet owners kept stacks of reagents to make new pets so if one died; meh. Any sort of healer could easily top off a caster if they got hit a few times.
However the same can't be said regarding a warrior/paladin/sk. The group game was very broken in this regard. Imagine a group of level 45-50 people in Skyfire armed with a druid or shaman that has greater healing. However there were also many tanks that asked to group with us; often we'd say yes. When a tank was invited of course things slowed way down due to game mechanics. Keep in mind at this stage in the game a lot of the general mechanics were still being figured out by the players; many tanks opted for max HP in lieu of AC. This is fine if the group has a CH cleric but sucks major monkeynuts for other healers. So once again tanks were pissed due to a lot of them either not getting groups or getting groups but abysmal xp rates. Many pet groups would of course opt for efficiency over all other aspects of the game in the race to 50/60/65/etc. which of course led to tanks getting pissed. This continued until druids and shaman were given better heals.
The raid game had other problems with pets. A lot of pet owners had very poor pet control. Pets also pushed mobs until Depths of Darkhollow was released. By now many people in the game had developed a pretty anti-pet stance. I first started raiding when Planes of Power was released. Our guild leader hated pets. One of our raid leaders openly bashed pet classes; other raid leader didn't bash us but didn't allow pets on raids. I moved to a different guild that allowed pets on raids but the membership would constantly use /gu as a "those dam pets are pushing the mob all over". One raid in particular pets were blamed for wiping the raid (claims he was pushed too far) when Fenin Ro was almost dead. I and all other pet classes decided to not summon pets for one week. You can probably guess that mobs were still pushed around but despite no pets out at the raid. Yet we were still blamed for mobs getting pushed.
Fast forward to today. A lot of the old animosity towards pets has simply been carried from one EQ generation to another.
The Five Monkey Experiment comes to mind when thinking about this.
The above is simply my experience. I've raided everything from classic through Depths of Darkhollow then off and on through Call of the Forsaken when I finally came to p99. The above has been pretty consistent from Saryrn, Rodcet Nife, Bristlebane and Fironia Via servers.