Cats aren't very good at catching sticks an dogs aren't very good at catching mice. Therefore you can't have the dogs trying to do what a cat can do....
Except design is different. As a developer, once you have worked with some PSD efforts a few times and then worked with real dynamic content then worked with a real client to get the thing to actually convert sales, then worked with the client stats you have got a good grasp of when something is right on the design front. Plus you can open the css and do something about it. Or you can open up some stuff on the server side or put in some hack with javascript to get the usability to what it should be.
Designers are completely out of the loop after go live, they have done their 'lorem ipsum' stuff. Proficiency at UX comes from delivering the deliverables, testing, testing and testing. Listening to the client and the customers. It does not come from a few static designs.
I have never heard of a designer wanting access to the css to make that 3px change. They are welcome to it. They would make my day if they wanted to do that stuff. They do design for the web and the css is really over to them if they want to do it. The problem is that they would have to learn what a border is, what a margin is and what padding is. It is not difficult stuff.
The whole point of CSS is that the presentation is a separate thing to the content. Any developer would be happy to churn out the content blocks and sensible markup and hand it over to one of these PSD experts for them to get exactly to their 3px requirements. But in the real world it does not happen. Developers can dive in and fix something at silly o clock on a Sunday, but would anyone ever call a designer at such times even if it was a UX issue that was the problem?
Obviously there are some fantastic designers out there - allegedly - but, in general, designers don't do the hard stuff or even bother to work with live content. Far too many of them produce static nonsense in PSD format - a trade that should have been pronounced dead the nanosecond the iphone came out. They also perpetuate this myth that all developers are totally retarded when it comes to anything to do with aesthetics. Yet it is the developers that implement design, learning along the way. Rarely if ever is it the other way around, and, should a designer dare to do that they will get clumped in as a 'front end developer' and have some retarded PSDs foisted on them by some manager that thinks only designers can have any input whatsoever regarding UX/UI design.
How often do you think a developer tests something? A given page can be tested 100's of times. Along the way 'this checkbox might not be needed' or 'this dialogue box makes no sense' or 'this process is tedious' gets discovered. The guys with their PSDs just do not have that insight.