It valuable to be able to remove 1 thorn in the whole boss fight, but not as valuable as being able to remove any thorn in the whole boss fight.
Lets put it this way: If I know the mage will never get the thorns… What I can do is place the earth shield (1 GCD, permanent -6% DR + 20% healing on target) on the squishiest party member.
So. For example, a rogue will take 3 ticks to die. So the first tick he gets it anyways because I begin casting. My buddy is a Dudu, that takes only 2. Because Rogue stacks Vers and Dudu does not. Thats why.
In practice: If the rogue get it, I can keep him healed with Healing Waves (slow but mana efficient heals). Because of the vers he has.
Without earth shield, the dudu will die in 5-6 ticks because I cant keep him at 100% permanently, and those 5-6 ticks are healed with Healing Surge (mana inefficient). With earth shield I can keep him alive indefinitely with Healing Waves, and will take 3 instead of 2 ticks.
If you take the mage out of the equation, with only 2 possible targets its EZ to heal that.
Now. A mage. Even with the 1000 defensive it has, he cant use them while he is staked. Like most of the classes. And because he does not stack vers and is a clothie (spikes = physical damage)… he will be in the same situation as the DuDu.
So now you have 2 squishy targets…
I cant stress this enough. The difference between having 3 potential targets, to 2 potential targets of the spike… is ENORMOUS…
And then on top of that you add RNG. Because you could get lucky and have the mage get all the skipes, especially after fire. But, the worst case scenario for RNG (mage gets NONE) is MILES better, as I explained (only 2 people getting the spike).
So even with bad RNG, there is no scenario that is worse than not having a mage. Not even marginally. And because of that its a cheese.
And I will give it to you that in lower keys its not technically necessary to have a mage for that boss. However… regardless of key level the fact that its MILES easier for the healer… remains. In a 26 or a 10. Does not matter.