It's justified, primarily, by saying you're summoning the essence of beings captured by the Well. Since I think Emperor Cole is the only Incarnate sharing his power to his minions (to futher his goals), they are the only ones upon whom you have essences to call forth.
As for the Grindy-ness, all I can say is I agree. It does seem to be rather repetitive, and it is pretty slow going. However, I think you might also be able to get similar incarnate XP and Thread rewards from the Apex and TinMage TFs. I can't say I know this for sure, but I do believe they've changed those two to be more in line with the Incarnate progression they've set up with the Trials.
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The Master said: "It is all in vain! I have never yet seen a man who can perceive his own faults and bring the charge home against himself."
>Analects: Book V, Chaper XXVI
As for the Grindy-ness, all I can say is I agree. It does seem to be rather repetitive, and it is pretty slow going. However, I think you might also be able to get similar incarnate XP and Thread rewards from the Apex and TinMage TFs. I can't say I know this for sure, but I do believe they've changed those two to be more in line with the Incarnate progression they've set up with the Trials.
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The Master said: "It is all in vain! I have never yet seen a man who can perceive his own faults and bring the charge home against himself."
>Analects: Book V, Chaper XXVI