Right... but it's a cult. Cults often expend resources, even when resources are scarce, in the course of ritual & to heighten the "experience" - I suppose it might even add legitimacy to seemingly-superfluous things (like flamethrowing guitarists) that they are intentionally done with the knowledge that scarce resources will be consumed in the process. If there's no cost to the pomp & circumstance, it might seem hollow?
If you watch movies looking for anything that might be anachronistic or implausible, I personally think you should focus on the bigger-ticket stuff that's integral to the plot. In this case, I think I've provided an adequate potential explanation that jives with anthropological understandings of cults... instead of a human sacrifice, it's like a gasoline sacrifice... an ostentatious showing of power meant to intimidate partly because of the resources it consumes. You also see this type of display in nature, usually as part of sexual selection...
And on that note, I do think the guitar (especially in light of its pyrotechnic ejaculations AND eventual fate) is a pretty transparent phallic symbol...
So the guitar is a wang that shoots fire... a wasteful, insane, indulgent display of masculinity and blind rage. I don't think that's a stretch, given the emphasis on what happens to it (one of the 3D effects, too!) in the end...
I'd love to get into a lengthy discussion about the ways the film could be interpreted as "feminist" - I put the word in quotes only because, along with "feminism", it tends to get interpreted at least two different ways. Anyone have thoughts on that topic?