Ok. That kinda makes sense.
Firstly though, ASLEF are being given that 'power', as you put it, because the company have failed on promises they made to enable RDW - in other words, they've cheated. They've said yeah, we'll do this, to shore up shortcomings in their management? And they haven't followed through.
Secondly, the advantage of these agreements is the benefit to everyone concerned - the company, employees, and believe it or not passengers. If you didn't have these agreements as an option, it would be similar to other industries, like you point out. Having them means you don't need to go into full-blown dispute - then things would be even worse for passengers.
So in other words, it's a means of getting things done without going into dispute. That's got to be good, right?
If the company isn't going to follow through on promises, they shouldn't be entering into agreements. But they do. To grab short term benefits. With no intention of honouring them.