The main reason for the guest network function is to allow non corporate users to access the internet without going across your core network. This provides a useful service (so your guests can use their iPads etc) without compromising your own network security.

Legally it is also quite useful to be able to say - we just act as provider of the carrier service on the guest network - as then responsibility for anything that happens on that network is almost zero.

Most routers that I see with this use two separate frequencies and two separate ethernet segments. I wouldn't use one that didn't have both of these - the first to avoid killing bandwidth, and the second to provide a gap between networks.