But in the case of discrete multi-channel, what you're asking for is the ability to literally create a personal of remix the front three channels. After all, the three channels are normally routed to their respective speakers with no use (or need) of processing. So what you want to do is redistribute the contents of those channels.

I agree that remixing the front three channels is useful when it comes to matrixed formats and is not "necessary" with discrete formats. However, I presume that, given the ability to remix for one (in the digital domain), it is trivial to remix for all. If that can improve support for 2.1, or 4,1 speaker configurations, why not offer it? I suppose one can argue that 2.1 or 4.1 systems are rare and so are not worthy of support, but again, if it's trivial to do, why not?

The bottom line is that I'd like the remixing capabilities to be orthogonal to the source formats even if they make more sense for some formats than others.

Remixing the front soundstage is not a solution to problematic speaker dispersion, any more than it is for a speaker configuration bug. Those problems should be dealt with, not covered up.

I would not consider cylindrical dispersion patterms problematic. In fact, they neatly address the problems of ceiling and floor bounce (for the mains) when compared to sperical dispersion patterns. A rotated dispersion pattern for the centre speaker is indicated as a matter of course. Remixing the front three channels would be a way to tweak the effect this has, but is not essential in typical rooms. I mention it as a possiblity, not a requirement.

The solution for you might be to actually buy an inexpensive mixer

No, the solution is a center channel speaker. (I think we'd agree here.) The BG 220i runs around $1800, however, so it will be a while before I purchase one. I would consider that a decent mixer would cost as much, unless I built it myself.

What specifically do you find "silly" about using PLIIx on 5.1 sources.

I'm of the opinion that the PLIIx processing would do more harm than good. I can understand PLIIx to go from matrixed two channel to five speakers and a sub. I can't understand the benefits of using it to go from five discrete channels to synthesize two more. I'm of the opinion that the resulting mangling of the surround channels as provided would do more harm than good. PLIIx on 5.1 to 7.1 strikes me as an excuse to justify two rear speakers instead of one.

I never associated "digital" or "analogue" as having any relation to something as arbitrary as number of channels. I've always viewed them as being orthogonal and really don't see what one has to do with the other. But that's just me.

History has a way of shaping one's perceptions. It's not so much the number of source channels, as it's the fact that matrixed channels were first distributed and decoded in the analog domain. So, when I read "matrixed", I think "analog". IIRC, the matrixing transform is not reversable, which means that encoding for matrixed distribution is a fussy, unreliable business, that also reinforces the "analog" adjective in my mind.