The thing is that if you let the umpires have more discretion you'll probably get a fairer result in aggregate, but people will focus on the times the umpire errs in his or her judgment and the umpires will just cop more guff than they do now.
That said, maybe now players can play-on while the penalty is being marked out they could bring back a 25m penalty for some of the more minor indiscretions, and keep the 50m for acts of bastardry.
And surely if throwing grass is an infringement – which I supposed it is – the penalty should be that you get the shot again.