I see no reason for more than 802.11n at this point. Why include costly modern wifi features that none of your customers are going to be able to use due to lack of a suitable router? A later more recent spec can be rolled out if needed, but n is perfectly up to the task of playing games and streaming internet content. Heck, 802.11g is! The only obvious interest in faster wireless is streaming video from the console to other devices. Streaming uploads to the internet will be capped at internet upstream BW, which is typically low (1 MBps).
I concur, 802.11n is more than enough for the majority of users, hell even the ones who can use the new standard won't see much use out of it as the servers Sony use are not up to the task of providing the download speed to those users.
The cost vs benefit is basically non-existent.