The way I see it, we get the opportunity to test out newly acquired gear AND practice against warriors (as for myself, since I can't find a PvE pug, it'll give me the chance to hack away at some tyranids for once).
Uh, I'm not sure what you're asking for here. An ability to take damage so you can test durability equipment? A way to measure how much HP the warriors really have?
Think of it as a firing range featuring tyranids. Also I've gotten several weapon drops from the lootboxes but have no idea how to apply them well, testing them out against tyranids would be great practice.
Which is literally what the garrison already has. There are no fewer than four firing ranges in the garrison and they have (admittedly static) tyranids in them. And those tyranids even have the decency to display damage numbers over them when you shoot them
I...um....*cough*.....maybe perhaps need to better explore the garrison rather than just take a peek next time around.... >.>
Fluffwise, I'm not sure if any of the garrisoning factions would be allowing wild nids running around. It's a major security risk, though then again, knowing Chaos and Orks in particular I'd not put it past them to try and set up gladiator pits to test mettel against Tyranids captured along the way. Crunchwise it's not a bad idea. Nids are constant enough foes, and the Garrisons aren't that laggy, so you could get accurate ideas at how your loadout is sitting compared to em.
I don't care how its done, Target dummies that hit you, swinging servitors, tyranids...BUT I FUCKING NEED DIS!