They don't in my book. I don't have rear speakers and plugged that whole panel with Dynamat to keep it from ratting when the sub hits hard, but the rear seat passengers can hear it well enough. Either way, not even the front passenger is likely to care about the soundstage from their seat, and those who have hitched a ride with me that normally do, accept that it's not their car and the same thing will be the case if I'm hitching with them. Plus, in our car audio meets, I always ride in the back at some point - not listening, but I just want the wind from the A/C as I sit in the middle - and I can hear it all well enough to sing to it if we were on a road trip (provided that car didn't have anything louder than a G-Medallion exhaust). BTW, some EMMA events use two judges on both front seats, so one can't completely rely on time alignment in those cases.
That said, the time alignment setting that really screws up the soundstage on one seat while improving it for the other is just optimization. If one gets the installation as right as possible in the first place, vocals would only be slightly off center for each and the rest of the stage just a little bit more screwed up (as it is in my car). And the great thing about on-board processors aside from not having a separate box (I'm looking at you, Alpine) is that I'm sure to see all settings on the screen instead of only setting it with a laptop then hiding the processor after tuning. With my (old) 860MP I just need a stoplight to disable the time alignment, or even switch to the setting to bias the time alignment for the passenger seat. I often use this to illustrate to those uninitiated to hi-fi why my tweeters are mounted as they are without stopping to swap seats.
Edited by ProtegeManiac - 11/28/13 at 7:16pm