If it's about your own system: why would you do that there?
Well quite a few people that I know of have computers in their household designed so their kids can't break anything, but can play games and do ... well, stuff. With increasing constraints come more and more issues which may arise, not to mention the simple fact that it's much easier when you can just work with a single installation as opposed to multiple. These systems were not designed the way they're designed just because it was fun to do you know,
First of all: no need to patronize me, I'm very well aware of the implications, as stated above.
So you're talking about preventing your kids from breaking stuff. How is that related to noexec? Without root access there's no way to break anything, except their own accounts and mounting a partition noexec won't even prevent that.
Granted, there are situations where you might gain root access via exploits but if your kernel is not patched and vulnerable to something like that you're in trouble anyways.
Those systems were indeed designed the way they were and that was: a multiuser environment were applications are executed with user privileges. Also, keep in mind that ANYBODY with physical access can compromise your machine at will. It doesn't matter if you're using Windows or Linux. You could simply boot from a rescue CD and chroot into your installation.