Much like Avengers and the AoE Rulers we're going to get some time into the future, their use isn't so much in killing the thing they're specialised against as it is in making farming certain nodes fire-and-forget.
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Though if the situation arises, of course (*cough* challenge quesrs *cough*) having a single unit that can deal supereffective damage to multiple unit types can be quite a lifesaver. Especially since both the ST Alter Egos we got so far strip away buffs when they do their thing.
Edit: Nvm, forgot about not-Illya.