Reagants.. Say you play a mage. You doing a t9-11 guy who AoEs. Their pet is useless as it will die in seconds. The only person that cares about the mage pet is the mage. What a hassle it is too get another one up swap spells to rebuff it. Cause alot of fights the mage cant rain mobs due to pulls and such. So mage then turns into a nuker. His single targets nukes drastically cuts his dps down with no rains nor pet. Before any flames come I do understand different mobs different strats and not any one class is going to make a difference. For instance I put the bst on the same page as the mage on those mobs but the bst will be dpsing and the mage casting but probably going to do the same amount of dmg if parsed.
You are not totally right here...
First, do your pet tomes. They REALLY make a difference.
Second, make sure your pet has gear (summoned silks add a lot of resists too)
Third, use barrier ward (removes 75% of spell damage on pet) on those named fights. Yes it has a too long recast to be really good, but chose what fight you really want pet to live.
Fourth, be ACTIVE with your pet... like call it in to you to get that group heal that has to be casted on the caster group anyway.
I have seen way too many pets die just because the owner dont call it in but let it stand there spanking away. If that is the case, let it die...
About using rains...
If you actually TALK to the tank about having mobs moved a small bit after mezzes, there usually is room to rain.
Most fights a mage can be good dps, a few not. (but every class have fights where they are lower dps)
On a sidenote.
On those heavy AE fights (if it's fire or cold) those 100 extra resist from elemental barrier can keep a caster group alive.