Mountain Lion will only be $20 for every computer in the house, and there's a lot of benefits to upgrading to it. Andrew wanted to use iCloud, and that's one big reason, but it's also going to be very secure with Gatekeeper. It'll be worth the wait, but it won't work on all Macs. The older his Macs are, the less likely they'll be supported. He won't have a problem if all of his Macs can't be upgraded though -- they'll all work fine on the same network.
Apple wants everyone to be using Lion, and soon they'll want users to be on Mountain Lion. Leo says he'll probably want to upgrade to Lion because the only service that iCloud works with in Snow Leopard is Mail. He won't get some of the other features without going through quite an elaborate process. There is an article from EggFreckles.net that will help him use iCloud in Snow Leopard if he still wants to.