Extend and enhance

I suppose the big disadvantage of web-based applications is the inability or difficulty of creating plug-ins for adapting or enhancing the functionality. For some desktop applications, that's a big plus. The only way for the average developer to extend or enhance a web application is if there is a provided application programming interface, but that is both rare and far removed from the desktop version of the plug-in architecture.

Mokalus of Borg

PS - Of course, with tools like GreaseMonkey it's possible to modify or extend web applications.
PPS - But it's not easy, not reliable, and usually against the terms of service.