True statement about RULE_INIT variables. The biggest difference is how the variables are stored and accessed. The static namespace is specifically designed for CMP-compatibility - meaning the variables will be accessible across TMM instances. A non-static global variable is not only not CMP-capable, but will likely demote the entire virtual server to a single TMM (not good for performance). So static global variables can be a good thing, if used properly.
By the way, while they are intended to be read-only once set, you can change a static global variable outside of RULE_INIT, but doing so will break CMP for that variable.