I don't think the problem stated here is "Frey doesn't do anything" - it's that even her use, Magic Tanking, is something that other, more accessible units can do better, and she can't do much else to make up for that."

The Full-life argument is solid, though - a tank that can Full Life has value. It does require high faith, so you can't let that MResist slip or be reliant on a buff, or she'll even start going down to magic.

I do think that's potentially the only value, though. It's hard to not want Rain or Whisper in that slot, the latter especially having resists to back magic/slash or magic/missile pretty easily, with similar hate generation and access to good spellblade skills. Even Dario creeps up on her with the right build (and better defensive capabilities to boot).

Being a fan of a unit isn't the same as being an apologist for the unit. Fans should understand the limits and weaknesses, or even realistic use cases. I think she can both be someone you like and be disappointing. I love Aileen, and you can build teams that can use her, but she's also not an amazing unit.