I've been asking myself the question for a long time. The answers provided here aren'T really enough to convince me - just as an example both the 767 and 747 are able to fly a GA with only 1 AP.
The point that the AP won't "see" that it deviates from its flight path doesn't really convince me either - think about GPS approaches using VNAV and LNAV with only 1 AP... Or simply flying a route in LNAV.
Regards,
Mark