An old friend asked to see my Nexus 7 at the weekend, they observed that ordinarily you either have to hover over the shoulder of someone who is playing with your new toy, or accept that everything on there (email, photographs, notes, contacts) is open for them to look at, even if they don't specifically intend to snoop. I had created a guest account and so they just got a rather bare desktop, it's not as easy to appreciate some of the device's features this way but it does avoid some potential for embarrassment or, in the case of devices used for business, data security breach.
Obviously the main purpose of this functionality will be for the larger tablets like the Nexus 10 that are likely to be shared by two or more people in a household. Most people aren't going to buy their five-year-old a high-end tablet, but they appreciate that the games and other activities are attractive and a child that age is capable of taking responsibility for treating the device with respect when borrowing it. Being able to give them an environment that retains the games, the camera and so on but lacks Mum's work email and a paused horror movie she was watching last night makes good sense. At least until powerful multi-core tablets really are given away in cereal boxes.
However there doesn't seem to be an easy way to share applications. If I have Program A, and I create a new user, Barry, there doesn't seem to be an easy way to let Barry use Program A, it's necessary for Barry to re-install it. So that's potentially a little annoying. But perhaps I missed some trick to doing this.