My 8 month old labrador hasn't got perfect recall yet so he is always on a lead in a public place. My old labrador, by the time he was 18 months and until the day he died, had perfect recall. He was under my control at all times when he was off the lead and I could completely rely on to do exactly as he was asked, the first time I asked him. I also understand that not all dogs are able to do this, no matter how well trained they are - my old labrador had a very gentle, calm personality and was very easy to train, he just always did what he was told and I completely realise that not all dogs are as easy as this, my current one isn't!
The law requires a dog to be "under control" currently and as I was correct in my belief my labrador was under control, there will be some dog owners who overestimate the level of control they have over their dog's actions. This leads to the current issue of dogs running up to people and other dogs and the owners have no ability to prevent this if the dogs aren't on a lead. I also find this so frustrating when I am out in public with my puppy on a lead trying to teach him that not every dog wants to play with him, thus desensitizing him to dogs and people meaning his recall improves. People letting their random dogs run up to mine completely defeats the object of teaching him to ignore them unless he's specifically told he can go and say hello.
If it meant the dogs who don't have good recall are kept on leads rather than being allowed to run up to anyone and everyone, I'd be all for it. But I would still be sad for the dogs who have been trained so well that they have the freedom of being off the lead due to the fact they'll always recall first time. These dogs, though, will survive and I think the benefit outweighs the negatives on this occasion. There are so many dogs who are fearful of other dogs and are approached by dogs out of their control, there are also people who are either scared of dogs or just don't want to be approached, children who can be knocked over by dogs running up to say hello - "under control" doesn't just mean "within sight" as some owners seem to think it does. It means that if a dog will not respond to you first time and if you do not have complete control of the dog's actions at all times, he is not under your control. If the training hasn't reached this point, a lead is the only way to have this level of control.