So question then... if it's so taboo to be driving speakers in series (which when you get down to it and think about it, yep.. I wouldn't either...), then why do we see soooooo many lofi and midfi receivers and integrated amps out there that do it when you select more than one pair? Even my M80 - it has connections for three pairs in the back. If you do A+B, A+C, or B+C, it'll drive in parallel... BUT.. if you decide to do A+B+C, then A drives in parallel with B/C in series (figure THAT one out... ).
I guess the answer is you might have a pair in the living room, another one deckside outdoors, etc... but nobody ever mentions that when you have two pairs stacked on each other, that you should consider how the amp is wired up to handle multiple pairs.
that could pretty well explain part of it. very helpful when there is little to no interaction between speakers, but I'm more of the impression that it's a "feature" to just sell more product.
but by the same token, there have been many Pro Sound Systems that have utilised series/parallel with no ill effect.
as an example, we have the JBL 4682 Line Array. 4x 10" drivers with a pair of JBL 2402 Rings, and to this day, one of my most favoured vocal foldback setup, (or even front of house for a very small place!)
and as an afterthought, thinking back on the Marshall Quad Box, when '
mic'ing' up, generally we (sound guys) usually get the guitarist to play a couple of bits, and then we '
mice' up the best of the 4 speakers, but it can take a bit to figure out which, as in real life usage, they all pretty well sound identical.....
go figure.
BUT as far as home use, it is un-necessary, and I generally avoid it.