Allowing players that are phased to attack other phased player is to assume that the nature of phasing for all those power sets that apply are the same. They are not. Some powers cast the target or user into alternate dimensions. Others might be shifted in space/time, etc. To allow phase players to attack each other is essentially saying that despite all the colorful descriptions that are provided for each power set and power, it doesn't matter, everyone ends up in the same closet when they leave the plan of existence that Paragon City is on. That's absurd. There's an infinite reasons to leave PC in the game reality, and the chances of two players that phase actually running across each other in that place and fighting is highly unlikely and equally absurd.
