I think that a roles debug tool that is accessible to everyone is dangerous
Even if it requires login? Even if it only works for the current user? If so, dangerous in what way?
My little script works only for the current user. To use it requires no special knowledge of roles and capabilities, just the ability to follow simple instructions.
I don't see any advantage in having "the ability to select any user to look at." If there is an advantage, what's the use case?
Here's the use case I have in mind: Someone posts to Moodle.org with a possibly roles-related problem. It doesn't matter if this person can assign roles or necessarily even understands roles! We ask them to (1) recreate the problem (2) use the tool (3) post the output back to the forum. I have already used it this way several times with good results, so it's a real use case, not a hypothetical one. No one had to give me a login to their system, and no one had to find "the the person who set up their roles" to help them. If only someone else could use the tool, they would have to duplicate the user's problem, and that would require the user to explain what context they were in and what operation they had tried. It's an inefficient process with lots of chances for miscommunication. As an admin I would consider it a waste of my time.
This is similar to the use case for texdebug.php. If people are having problems with the TeX filter, we tell them to run the script and paste the output into a forum post. And texdebug.php doesn't even require a login!