the Jester
Legend
It is much more logistically difficult to juggle and have an overall positive outcome for everyone involved. And once even a small core group of the PCs survives to a decent level, they out of necessity need to be all-but retired; including even a single PC of significantly lower level immediately skews the session.
I agree that it requires some logistical creativity, and maybe it's just that I've been running my game like this for decades, but it's not that bad. I mean, I don't need to keep track of story elements and trying to make sure the party isn't straying too far from a story or anything, so I guess it's just a matter of where your preparation/energy/focus goes and what suits your style.
I do think you overestimate how much of an effect having mixed level parties has, and I think you also underestimate the importance of attrition when it comes to dealing with lower-threat monsters. But then again, I use a slower recovery variant for regaining HD, and that probably influences my perception of the flow of it all.