The problem is that simply having an IP address that is accessible doesn't mean you are a node. Proving you are actually a node especially over an extended period of time is a non-trivial problem.
Absolutely true. And thereby hangs the crux of the problem of motivating people to keep nodes up.
It's far more important that they be reachable via separate network paths, don't all go up and down at the same time like dozens of virtual machines on one server, and don't actively collude to subvert network security. In fact, it might even be as problematic for security as pool mining.