On top of everything everyone else said, it also sounds a bit... what's the word I'm looking for here... cultish, maybe? Imagine Objectivists with a forum asking "What's the most Objective toothbrush?" or "How should an Objectivist deal with situation X?" Or "How should a Scientologist deal with situation X?" The connotation here is cult like - "How should my in-group, which you and I are a part of but everyone else out there isn't part of, deal with situation X?"

This doesn't mean using the words "rational" or "rationalist" in this fashion is actually cult like, but it's good general policy to avoid looking like a cult - especially when the cost is low.