It's simpler than that. ARM provides the whole CPU core design. If the spec sheet says Cortex-A9, then it's Cortex-A9, no matter what customization have been done. Customization pretty much means that you can add e.g. video decoding/encoding engine or something similar, and you get to choose the GPU and the amount of RAM. The CPU cores don't change, only their frequency does.
Sure, you can still use ARM instruction set and do what e.g. Qualcomm has done with their Snapdragon lineup. That would be a very risky move, though. That requires years, even a decade, of R&D if you want to be competitive. You don't just hire a few engineers and they will come up with a new chip in a few months.
To be honest, I don't see Apple going that route. The end-user doesn't care what's inside as long as "it just works". The SoC market is also fairly crowded now that Intel is joining the party as well.
Why sow your own seeds while you can just wait for others to do it and grab the fruits?
I think that is the problem. People will be hesitant to switch for ARM based machine if it means lots of compromises.
Also, I don't really see what ARM would do better than Intel. I see lots of claims about battery life but in all honesty, that's 100% speculation. The ARM devices we have nowadays don't run full-blown OSs like OS X - they run OSs that are designed for more crippled hardware. There is no real multitasking for example. I can't have three browser windows open side by side all playing Youtube. Hence you can't compare the battery life fairly, OS X needs way more resources to begin with. Heck, the RAM requirement is 2GB in Lion while the most we have in iDevice is 512MB.
Moreover, the ARM chips we have today are designed for devices like phones and tablets - not for laptops and desktops like Intel chips are. We really have no idea how ARM would do at the same performance level. I would claim that ARM is way behind in performance per core and watt. Adding cores and frequency works to some degree but at some point, you will hit the wall. Software is extremely slow to support multithreading. We have had quad core CPUs for over 5 years but the support isn't overwhelming, let alone when we go past four cores. Thus adding more and more cores doesn't work, and that's one of the reasons why Intel's Ivy Bridge is still quad core. Also, at least for Intel's chips, 4GHz seems to be a magical obstacle. When you achieve 4GHz, the power consumption start to go up
substantially. I can't explain the reason behind that and whether it would be the same with ARM but it's partially due to current leakage and the physics of silicon.
An ARM based MBA wouldn't run at 1GHz like iDevices do. We should be looking at least 2GHz, possibly closer to 3GHz, so the performance drop from Intel wouldn't be totally laughable. We don't have any ARM chips at 3GHz, so we don't have any idea how they perform and how's the power consumption.
Okay, this got a bit long so I'll summarize here: The pros of ARM are more or less just speculation. There aren't any ARM chips that can beat Intel in performance. Performance and power consumption don't always scale up nicely. You can have a chip that is very efficient at 1GHz but not at 2.5GHz, or vice versa. Also, if Intel's Medfield is as good as it looks, it would bunk the ideology that ARM is superior to x86 in terms of efficiency.
