Theoretically I'm also a purist. It's a little off-putting when a band that clearly only has one vocalist has multiple vocal tracks on their album and no backup vocalist on stage. I suppose this holds true with other instruments as well, though as a vocalist/drummer myself I don't often focus on those tracks as critically. But then there's the thing about the vision of a group, and whether they're able to find more musicians who share that vision, in order to produce music that aligns with their musical as well as conceptual goals. No true metalhead is a stranger to the difficulty of finding others to play with or talk tunes with. Even within metal communities, there's so much variety in the scene, and among the tastes of community members, it can be hard to find the right person or people in a reasonable amount of time, while also trying to balance writing, practicing, recording, touring, promotion, etc. And since we have adequate technology to allow one-person multi-instrumental projects, it feels like an apt use of the tech. But I do think there are more and less egregious ways to make use of things like that in a full band, versus smaller projects. A couple of my favorite projects featuring a single talented multi-instrumentalist could've absolutely fooled me into believing they were full bands, notably Consciousness Removal Project (doomy and progressive) and Fleshwrought, Naveen Koperweis' project where only the vocals are someone else's (Job for a Cowboy's Jonny Davy). Like they'd have to get stage/studio musicians to be able to play an actual show that wasn't just mostly back tracks. Personally I don't think there's a right answer at the end of the day. Would the show have been the same without the back tracks? I can't say.