I find that URPGR also cuts down on the length of the rules, also; rather than repeat what's often explained for graphic content limits, these three numbers explain everything. =D
Back when I ran public games regularly, I used the R rating, or This is for a mature audience: adult content is prevalent.
And you're right about that: a lot of games I find turn out to be PG. There was an old discussion at RPG-D on ratings; high ratings are popular because some staff use it as it makes their RPG more 'elite'. The game's more appealing and attracts more members if it has a shiny R: For mature audiences only label, as opposed to a PG/PG-13. Which implies it's for kids, it's squeaky clean, which is far from the truth. You can get away with 'a lot' of graphic content on a board host, with PG-13.