Well, there's the real Showtime on stage careers and then the street performers. If the sims you are seeing in these slots are homeless NPCs, I would say that they are meant to be street performers. Even with NRaas StoryProgression denying sims both these careers and skills, the game itself will still shove other homeless sims into these slots only more slowly. It's not SP doing that and the mod can't really prevent the game code from doing so, it just tends to let them stagnate that way. I usually see around 2 or 3 in each one -- singer, acrobat, magician. Any more than that and I will either get rid of the sims (MC's Totally Annihilate) if I have no idea who they are or I might try to find something else for them to do, such as give them a cash register or bar to run by way of the Register mod.
For me it depends on the world. I don't want huge rosters of homeless street performers in Uni World (maybe a few would be okay) nor in what would otherwise be a sparsely populated farming or rural community. But for more densely populated areas I can tolerate a few more of these, not that I usually see them on screen actually doing much.
MasterController run from City Hall or an in-game computer will give you lists of sims filtered by career and you can use its Demographics Reports to get top level and more detailed information about what all of the employed sims in your town are doing.
