Some people like buying Ferraris and then never driving them, or babying them when driven and never putting them on the autobahn/track. But it still makes them happy to own one.
If a person is happy with a non-jailbroken iDevice and they are happy with it, then yes, they are using the device to their full potential.
Jailbreaking is about choice. And some people make a choice to not jailbreak, and that choice should be respected. To say that someone SHOULD jailbreak for the sake of doing it is, IMO, almost as bad as the "walled garden" philosophy that the jailbreak community rallies so hard against.
"I don't see the point" is a perfectly valid reason not to jailbreak. That should be just fine, as long as the person isn't refusing to jailbreak purely out of FUD reasons.
That said: I've jailbroken before, and regularly jailbreak iPads and iPhones for friends who ask me to do it on theirs, if they request it. But I don't push them to do it. And, I currently don't have my iPhone or iPad jailbroken, because I'm good with they way they are now.