I have had the opportunity to audit a few small business networks. The security of the network is often not even thought of. At the very least, you really do not want to allow your guests onto the same network that your point-of-sale systems are attached to. That is, the computers handling the payments. Even if those are connected via ethernet, they can still be logically on the same network.

Ubiquiti's guest networks, in the few that I have tested, were truly separated from the main network with out any additional configuration. They're worth the money. The other small business equipment will usually spin up an additional wireless network, but someone with the right know-how can still reach into the main networks and attack them just the same; Guest networks are often a free entry point.