- development cycle is easy and the tools are there,
- css is way more flexible then having to spec all padding and margins in code,
- graphical designer can focus on ui design, you focus on code.
2 years ago I was cursing against it but now, with html 5, they're actually making developers life simpler. There is still the problem that you have to design the look of your application while using toolkits you don't have to but then there exists design that you can reuse and it might also be considered as a plus by some.