I think it would take away a lot of the confusion and level the playing-field. This way you'll know who the good starters are, and who are the sprinters who are genuinely quickest over 100m or 200m. You do get athletes going off too quickly because of nerves. Discretion for officials I'm thinking of Iwan Thomas in the 400m final at the Europeans last time around. But there I hope the officials would be given the discretion - and even having said that, if athletes knew they were out if they went off early, even nerves wouldn't make them.
Swimmers are nervous, and they're held in a much more balanced position on the blocks.
You're still allowed to put your hand up if you're not ready. Think of the number of disqualifications we have anyway, for two false starts. Linford missed out in the final of the Olympic Games in 1996, as we all know. The key will be to get good, rhythmical starts, with a smooth progression from 'on your marks' to 'set' to the gun. In swimming they use an electronic start - and I don't see any reason why we can't move towards that fairly quickly.