Have you tested that? Or are you just assuming it.All allies within the radius of the effect. It's so high because it only applies to sneak attack damage, which is a small portion of the damage involved in most battles. If you are a sneak-burst-based build, consider developing some sustained battle tactics for when the one-shots don't work.
I have not run logs, but running RM in groups has elicited word of mouth feedback that allies within the area of effect have enjoyed the mitigation. Granted, this is not solid evidence, but I am ok with the results.
Also, look at it this way: Enemies have to take up one slot on EACH bar just to defend against sneak attacks. It serves NO other purpose. Your sneak attacks are very useful in many situations and grant you DPS potential across the board.
Who is winning here? The build that has to take up two slots just to reduce damage or the build that can just move on and apply damage elsewhere?