I'm not a network expert, but I think there's some advantage to getting all your devices on the same 'n' network using the same frequency. Otherwise you're dragging down the speed of the network. (I think.)
I'm pretty sure Apple's solution is to use the new Airport which can create both an 'n' and a 'g' network and you can assign the proper devices to the proper networks in order to keep your 'n' as fast as possible.
***There are probably a few errors in this post. This is how I understand it, though.