Now that web connectivity will likely require some form of consent from the user when using Udon, I think it's a reasonable request to ask for an actual unique user ID that we can use to identify users. This could be used to do things like enable a debug console in the world, or when web connectivity is back, save progress or settings in a world.
Right now people are using display names as "unique" identifiers which isn't good and already has issues since it's a display name. One issue is https://vrchat.canny.io/vrchat-udon-closed-alpha-bugs/p/onplayerjoined-doesnt-get-full-displayname-for-steam-users
Another issue is that "special" characters like "." get replaced in the display name with lookalikes which breaks string comparisons.
Relying on display name to "uniquely" identify people will become more prevalent as time goes on and people make commonly used systems that depend on them. You should expose this sooner rather than later to avoid this issue.
The potential for abuse is obviously possible, but there's no reason to kneecap the systems and introduce potential issues by leaving only display names visible. Right now I can reasonably target people with just their display names, it's just possible that there will be some more people with the same name that get targeted. Worlds abusing names to harass people is something that should be reported on a world basis.