So, are center speakers over engineered and are they actually detrimental to a surround system?
Yes and no and yes and no.
First, one of the great myths is that because centre speakers carry a lot of the on screen action and dialogue, that they need to be some sort of uber speaker compared to the pair standing either side. All of the channels of are able to carry the same maximum levels of sound and cover the same frequency range, so the truth is that ideally, all of the speakers need to be equally capable.
Second, yes a poorly designed 'dedicated' centre can sound worse and simply by dint of being larger with extra drivers, is unlikely to sound different to the stereo pair, thus undermining the whole point somewhat. That said, a well designed unit can pull that trick off, but arranging two (or more) drivers horizontally, especially where the upper frequencies are concerned, can cause some weird effects as you start to move away from sitting dead centre. It's much the same effect as standing up to listen to your main speakers where you change the relative distance of the two drivers if not the actual distance by much. This is why you will see that all of the really good centres have a vertically arranged treble/midrange array of drivers, with only the lower, less fussy frequencies being left to a pair of bass drivers.
Again it's not gospel, but the point is just because a speaker is sold as a centre, doesn't necessarily mean it's actually any good, nor that turning an identical LCR speaker on it's side will be a total answer either. The only array that is guaranteed sound entirely consistent across the front is three identical speakers at the same height, but that equally doesn't mean there aren't some excellent centres that are carefully designed to work well in their roles.
What is a shame is that too few manufacturers make speakers available singly for you to make your own choices. The cynical might say that there's more money in a a £500 pair plus a £300 centre, than in a £750 triplet if you catch my drift.
This subject is a personal pique of mine. I ranted about it in
my blog yonks ago and I haven't changed my mind much, but the one thing I forgot to include back then was the biggest myth of all - that
everybody needs or will benefit from having a centre speaker.
Russell