No XSLT was an attempt to make the UI generic, either you accept that UI are a custom tailored to be optimized for the platform or you accept the dumb terminal philosophy where you try to generalize the UI to adapt to all platforms.
The fact remains that human preference is for tailored UI and not until AJAX took hold was the desktops dominance as a platform threatened. Time and time again, tailored UI's are preferred. It is the reason for Apple's return to stardom as well as the rise of web 2.0.
Developers try to fight this mentality to work in their boxed solution and I understand it, but you have to clearly refute evidence to the contrary to believe that the user prefers application developed in this manner.
So If that is the case why not design an architecture that accounts for this reality and makes peoples jobs whose responsibility it is to assist the user instead of enforcing a developers domain on them.
As well, I do not agree that different UI's require different back end logic they require different work-flow logic and work-flow is not reusable. If they require different back end logic for say getCustomer, addCustomer, validateAddress then there is core architectural flaw in the solution. How a customer is then displayed is a problem for the UI e.g. Do I show all info in one screen do I show pivots to make room for more screen real-state.
It is not Apples fault, that is faulty logic, Apple is giving their users what they want and their last quarter clearly reflects that.
Web sites that rely on web services are not tricky to scale look at my document on amazon I referenced. It is essential to scaling. Google, Amazon et. al. are doing this precisely for that reason scalability and to not be locked into back-end or front end technologies. Not to mention you are distributing all work-flow and dynamic templating to the clients, which in practice has a large impact on resource utilization.