they have to take into account the battery consumption as well.
IMO the 13" rMBP doesn't seem like a laptop that is catered for professionals, but rather the "ooh i got a retina screen" type of consumer, doesn't seem like there is a need for quads until it the TDP is low enough to match the dual cores.
Power draw at idle from a quad core is LESS than a dual core. Consider this. A dual core's tdp is 37 watts, while a quad core's tdp is 47 watts. However, at idle and at low intensity stuff, only one core is in use. So, 37/2=18.5watts for a dual core, but 47/4=11.75watts for a quad core.
Yes, I know TDP does not equal power draw precisely, but the illustration is logical and accurate.