A lot of magic stuff has marked as non-spellstealable.
Spellsteal often gives you nothing (a huge HoT effect while you are constantly at full health? Right, shitty) except gutting you down for constant usage of it.
Yeah, you can easily get 100k absorb or bloodlust without negative effect on trash packs, no one really complains about it.
Mages do get punished for it, it's it mana cost and GCD loss (ever needed to spellsteal healing buff from 2nd boss in blackrock when you have 1,5 pyroblast available and you just finish casting fireball? Yeah, you often lose that extra pyros that could end up with 5k+ combustion, but who the fuck cares? Mages are dispellers now! I'll probably stay in green ooze and top healing meters for fucks sake)
I won't comment on how much these 6% is for Arcane, but for fire or frost, indeed, it's not that much. Unless you have to dispel every 10 seconds, then after 5th or 6th dispel you won't be able to cast a fireball for at least 2 seconds, or throw bloodlust+fireball+inferno+pyro+combustion combo because it eats away 25% of you mana in a short time.
And for this bloodmaul example you bring, these is stuff to dispel only if someone fucks up and skip interrupts of huge nuke (not really an issue with blood DK or a warrior tanking, since you can get 10% damage buff and DK can steal this spell to throw it back at boss). I remember thinking like you when i've done this HC fight for a second time, i asked to not interrupt his spells so i can get these buffs. I've run out of mana to continue my rotation after 5th spellsteal as fire mage. It was just 4 seconds downtime (notice that i don't have mana to 6th spellsteal, notice that i don't have mana to instant pyro, wait, fireball, pyro, inferno, no mana for pyro, wait, pyro, no mana for combustion, wait, combustion is too late, fireball, rotation is OK now).
It's not an issue when you have to dispel every 30 or so seconds, but these is a lot of classes that suit for doing this much better (shamans or priests for example)
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That's the same attitude i have on any class that can dispel, including mage, you don't spellsteal to get enormous benefit like you used to in WotLK for example, spellsteal got the same treating as mind control, it's still fun and somehow viable in PvP, but it's not that good as it used to be, It's just very expensive dispel. But when your group
forces expects you to dispel things that marked as non-spellstealable (One situation i've read on priests forums in CM everbloom 2nd boss, he claimed that a mage was on dispel duty, while all magic stuff that exist in encounter marked as non-spellstealable AND they had TWO priests in a group, while almost all these buffs are completely avoidable) it's just infuriates me. Also you kn ow who the fucking best dispelers in the game? Fucking glad/prot warriors with stupid major glyph on their main ability, boom, no need to dispel anything, problem solved.
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Because things like 100k shield absorb or a HoT effect on a class that isn't supposed to take damage makes so much sense and worth spending GCD and extra mana, right.
Seriously, if the spellsteal could be glyphed to decompose the spellstolen buff and get different effect like spec exclusive proc or raw damage or to get your mana back over time it would be much better, but right now - this ability is shitty and very very niche. There is only two things iirc that benefit mage for spellstealing it, the 10/20/huge aoe buff from one of bosses and bloodlust from trash packs in the same dungeon. There is no buffs like
this to steal anymore, no need to overestimate the effect spellsteal provides