The kuroko appearing was part of the point of how the henshin and roll call was shot. Shinkenger is loaded with references to traditional Japanese theater and in such traditional theater (like kabuki), the kuroko appear to move scenery and props around onstage, particularly during a scene change. They're also part of bunraku puppet theater where you'd see them constantly.
The term is frequently translated as "stagehand." They wear black to indicate that you should regard them as "invisible," not part of the story they're telling, just helping move things along. The kuroko appeared in the Shinkenger henshin to indicate that the theatrical, flamboyant part of the show was about to begin.
The kuroko effectively escort the Shinkengers out of the mundane reality of the show's "talky" half and into the heightened superhero reality of a given episode's action-oriented climax. Basically they reinforce the idea that the henshin is in fact a major "scene change." If the villains were winning, they won't be in a minute or two.
If you reserved the kuroko for "special occasions"... well, the entire metaphor breaks down. The henshin stops acting like a "scene change" and you lose the idea of the Shinkengers being escorted into a given chapter's climactic scene. You honestly lose any real reason to use them at all, since in kabuki and bunraku they're going to be onstage very frequently.
This is all Shinkenger, of course. If the kuroko are in Power Rangers at all, they'll probably be presented as something very different (since most American kids aren't going to be familiar with kabuki and bunraku like their Japanese counterparts). I'm also sure PR will drop the kurokos' role in the transformation since it won't make any sense to most Americans.
