Why wouldn't I? I could get cards that supported the protocol for my existing equipment.
The issue *there* is why would you need the support for .ac now - if you have nothing that supports the protocol.
If I was buying today I'd get an AC router, because you almost never replace a working router...the cards can come later. Fortunately I'm not.
And yes, AC is on top of N, just like N is on top of everything else. 5ghz instead of 2.4 though.