I don't think it's an active waste of time to explore the research that can be done with things like AIXI models. I do, however, think that, for instance, flaws of AIXI-like models should be taken as flaws of AIXI-like models, rather than generalized to all possible AI designs.

So for example, some people (on this site and elsewhere) have said we shouldn't presume that a real AGI or real FAI will necessarily use VNM utility theory to make decisions. For various reasons, I think that exploring that idea-space is a good idea, in that relaxing the VNM utility and rationality assumptions can both take us closer to how real, actually-existing minds work, and to how we normatively want an artificial agent to behave.