That move takes away a bit of Mitsuru's lifebar every time it's used- if anyone is stupid enough to spam it like crazy, they're just going to make it even easier form them to lose.
Well it takes blue life with regenerates and the same goes for everyone's DP. Players on the verge of awakening will use a DP to get to that point sooner so their are cases when you want to take blue damage . Also since blue damage doesn't kill you players would probably be more liberal with their DPs.