An impact player is a player who can produce a bit of magic to change a game i.e. Shane Williams. Don't think they should include them though, it should be based on stats alone. For example, I wouldn't class Adam Jones as an impact player, but he's one of the best tight-heads in the world and a key player for Wales, and his stats should reflect that, without the need for a silly star above his head.
I like the way impact players were handled in the NCAA Football games. An impact player receives no stat boost or special skills. Every team has three impact players (you could make it two for rugby of course, that would look better), the designations just there to let you know that this is a guy who you want to have involved in the action right now.
For example
Ireland: Heaslip, O'Driscoll are both guys who you want to have the ball
New Zealand: This would be tough to pick two, I'm not a NZ fan, but I'd have to say McCaw and Carter
USA: Clever and Ngwenya (sp?) they obviously aren't as good as players on other teams who aren't impact players (like Sexton or Nonu), but this lets you know that when you play as the USA, these are guys who you can rely on, at least more so than other players.
This way the star serves a function to people playing the game, while not adding some unrealistic stat boost