This is something that's been going on in the competition side for years, but is starting to gain traction in the defense area lately. Caleb is playing around with a Glock with a Burris Fastfire II mounted on the slide, and it sounds like he's getting some more options to try out soon.
The biggest advantage in my mind is being able to maintain focus on both the target and the sight and the same time. In a defensive situation, being able to keep the threat in focus while maintaining good sight picture is simply not going to happen with irons.
Currently the biggest hurdle is sending off the slide to be machined for whichever optic you choose, but there are a couple guns coming from the factory with cuts, and Brownell's offers a drop-in Glock slide that's already cut. I'm hoping it'll gain momentum and become a more common option, right now the price is just too high for my budget, despite how much I want something set up this way.