Allowing soccer kicks and stomps definitely adds excitment, allowing the referee to "ok" it first is a way to make it somewhat safer (insuring that a fighter that is not intelligently defending himself / herself does not get soccered or stomped)
I believe the "open attack" rule can and will work once the fighters become more accustomed / aware of it.
The Cage Rage events used the same exact rule "open guard" for years and had very few problems with it