Either if it's for performance, battery life or cost reasons, it wouldn't really make sense:
a) performance wise, they move would be driven by having a better performing A chip
b) if they aimed at a 15W part battery life would suffer. 6W parts don't deliver good performance.
c) for cost, they'd have to buy the intel processor, and the infrastructure to support it (socket, chipset, heatsink, etc)
Specially for (c), I don't think either Intel would accept selling chips as co-processors (it'd be like admitting their processors aren't good enough to be main processors), nor Apple would put itlsef in a position to adjust the internals of their computers just to acomodate something which they are trying to get away from.
Apple probably doesn't need the integrated GPU, so an AMD-based coprocessor could trim that off for additional power savings (making room in the power budget to re-add hyperthreading or additional cores and/or to bump up the base or burst clock speeds).
> for cost, they'd have to buy the intel processor
Or AMD.
> and the infrastructure to support it (socket, chipset, heatsink, etc)
Laptops (at least the ones as thin as Macbooks) haven't used discrete "sockets"... ever, I'm pretty sure. The vast majority of the time the CPU is soldered directly to the motherboard, and indeed that seems to be the case for the above-linked APU. The heatsink is something that's already needed anyway, and these APUs don't typically need much of it. The chipset's definitely a valid point, but a lot of it can be shaved off by virtue of it being a coprocessor.
a) performance wise, they move would be driven by having a better performing A chip
b) if they aimed at a 15W part battery life would suffer. 6W parts don't deliver good performance.
c) for cost, they'd have to buy the intel processor, and the infrastructure to support it (socket, chipset, heatsink, etc)
Specially for (c), I don't think either Intel would accept selling chips as co-processors (it'd be like admitting their processors aren't good enough to be main processors), nor Apple would put itlsef in a position to adjust the internals of their computers just to acomodate something which they are trying to get away from.