The reasons are obvious: You
can change the backend or the frontend when you need to. You build them in parallel. You can have various different interfaces to the same content (mobile, web, iPad...) You give third parties access to your data and not to your system. You crowdsource innovation.
Using YQL has a lot
The elevator lift pitch YUI
is the system that Yahoo uses to build its web sites. It is constantly tested to work for the largest amount of users, free, open source and covers everything from design patterns to out- of-the-box widgets. It is modular and you can use only what you need. You can either host it yourself or get it from a network of distributed servers.