As far as I can see, the inevitable result of having several different root passwords across your systems is that a smart intruder who compromises /bin/su on a single system will sooner or later collect all your root passwords.

(Or perhaps other people are better than we are at typing the right root password at the current Password: prompt.)

(Please do not suggest sudo as the solution. For a start, we'd still want actual root passwords that are distinct from our own passwords.)