There's still a bunch of devices out there that use b/g, like my Kindle (the one with the keyboard) and older iPod Touch's that haven't died yet. For that reason my wifi network still allows b/g/n devices to connect instead of n-only.
Um, surely if you are using a b/g/n router then you will have no issues with airplay as your faster Airplay devices will use n and your older non Airplay devices will use b or g. In fact if you think about what you have just said you will realise you have just confirmed you have already invested in the very upgrade the others have suggested.