The DE book has some weird FoC choices. I don't know if they did it on purpose, but it strikes me as odd that there are 2 units that require FoC changing characters to make useful.
Why would you ever use Wracks or Hellions outside of the troops slot?
Wracks are pretty durable, but they don't actually do anything. Anyone ever taking them in the elite slot is wasting points. Imagine Plaguebearers that couldn't score and that's pretty much what Wracks would be without a Haemonculus. Maybe if you had 30 spare points, and an empty elite slot, they could be a token battery for an HQ. I can't imagine that ever being the case though.
Hellions aren't quite as bad as Wracks, but I just don't see the point in taking them outside of the troops slot. They aren't so bad that you wouldn't be hurting yourself too much by taking them in some sort of themed list, but in general any of the other FA options would likely be better. Even as troops they are still fairly lackluster, in my opinion, if they aren't accompanied by the Baron, but then they are scoring ASMEq's which is at least better than non-scoring ASMEq's.
It's just odd that they would put these units in the book in FoC slots that nobody should ever use them in. I mean, I sort of get it, I guess. If you just make Wracks troops, then it frees up 50+ points and an HQ slot, and the same can be said for Hellions. (although, I'd say the Baron is a bit more useful an HQ not counting the free token... ok, he's an actually killy HQ, let's put it that way) It's a balance issue (which is actually pretty refreshing to think that GW went to that extent) and for the most part, I guess it makes sense. It still just seems weird to me.
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