I am actually doing the muelleo, but my definition may not match yours. Muelleo is springiness -- a constant springy bounce in the knees. There is also a bit of weight going up and down, though due to the geometry of the body motion when the muelleo is small the head barely moves.
This is not just an issue of semantics. Just picking up and putting down heels does not reproduce the effect. When doing the muelleo, there is a definite spring in the knees -- my body going down and up on the knee joint as a car goes up and down on its springs. It's small enough that it's hard to see clearly from the outside, but trust me, on the inside the difference is 100% clear.
I do insist that the difference is there. Take a look at this video of Domingo Pau, a legendary figure of Cuban rumba, who does _not_ show substantial muelleo in the first part of the vid (at least none that can be seen from the outside). He does the heel up and down but his weight stays very stable and there is little springiness in the knee (I mean this in a technical sense -- a spring produces a constant & smooth oscillating motion around its position of rest). Toward the end there are a few moments where there's a hint of muelleo, but overall there is very little that I see:
Contrast this with the first guy in the vid I posted above:
His head also stays fairly stable, but his knees & weight are in a slight but almost constant spring/bouncy action (interestingly -- he doesn't lift the heel much to achieve this).
I'm not saying one is better or worse than the other -- Domingo Pau is a master, and the other guy (who probably takes classes from Pau at the Conjunto) looks pretty good to me too. They are both dancing rumba but they are doing so quite differently.
If the definition of muelleo is the dancing bear step, then sure -- that doesn't really belong in rumba. That is, if your point is that beginners are not being taught in the best possible way, then I'd say -- possibly so. But I do figure you have to start teaching an exaggerated version to make it stick -- you can't just start with the small motion. All except very, very talented students will not pick up on the subtle version. (It doesn't have to be the dancing bear version, though).
Which is DEFINITELY true -- and which is a problem I do see in Europe -- is that teachers are not doing their job in terms of explaining that the dancing bear version is not the real thing and looks ugly. Which leads to a lot of. . . eyesores at festivals and socials.
Re. your note about women -- that's something I've wondered about, actually. I've definitely seen women dance with muelleo, but aesthetically you may be right -- a clearly visible muelleo typically it doesn't go that well with the female version of the dance.
There are plenty of differences between how men and women dance rumba, though -- the basic step is similar but not quite the same. An example relevant to this thread -- one ISA prof told me that women should not pick the heel up from the floor like we've been talking about while men can. Perhaps this also explains some of the difference in muelleo.