I'm very concerned about MaxAlt's statement " ...any RA....lead to injuries...".
How on earth do they teach you, old chap? What software version are you using? Surely you don't fly your RA responses so aggressively that you injure people? I always stress smoothness and follow the commands. Nothing more. Anything else could lead to far greater problems, such as loss of control.
I'm not happy with the VFR traffic situation in class B, but as posted earlier, its legal. It's just different from the UK, where light aircraft flying has been all but neutered. In the US, it's a powerful lobby. In addition, terrain constraints affect many areas in the US most populous areas, such as SOCAL, where some form of VFR north-south solution had to be found.
I find it strange your mate didn't get a mental picture of the Cherokee's intentions and whereabouts from a listening watch on the radio. I do believe the PA-28 would be on the same freq in the same part of the class B, thus giving one a mental 'heads-up'.
A good lookout is still a potential lifesaver, something I think many of us have let slip in the modern age.