I have been active on Silverlight forums for a few months now and most of the developers I see have problems with understanding that Silverlight is not like traditional web development environment. And the approach they have to take frightens them because it is not traditional. They have fixed a development mind frame with with HTML/Graphics and Server pages and sometimes it doesn't suit well with the Silverlight architecture. There are some areas of web development that with the introduction of Silverlight architecture get void, and I am set to make points of all of them and discuss the reason. Instead of doing all of them in one post I'll try to post multiple entry concentrating on a single method each time.
The most basic traditional method that developers gets wrong, Silverlight doesn't support Middle-Mouse button, Right-Mouse button or Double Click and developers have expressed this as crippled functionality or nonsense among other things, But the question they ask why only one mouse button is to an extend valid, and I'll try to shed some light on the reasons.
To answer why, we have to answer why Silverlight? Why not AJAX? What Silverlight brings to the application that AJAX doesn't besides easy of development? I say cross-platform compatibility, but they say AJAX is also cross platform compatible, I say with Right Mouse, or Middle Mouse button it isn't.
Ok, but what does this have to do with Mouse buttons? The reason is Mac with its tradition doesn't have more than one mouse button, and this button in Windows terminology is interpreted as Left Mouse button, however in the last few years user have adopted the Mac-Might Mouse which have more than one mouse button, but the traditional Mac have only a single mouse button, so if you create a web application that is cross platform you have be use that all the users irrespective of their platform Mac/ Windows, or browser IE/FF/Safari/Opera.
You would ask what about double click? I say how would you double click on Mobile device? Not all mobile devices that access Internet are touch screen, and even on touch screen its hard to double click, when working on Silverlight you have to think the big picture, not traditional windows method. Microsoft have showed interest for Silverlight on Mobile devices but as yet, no particular announcements have been made. But expect to change that with Silverlight 2.0 RTM (by summer 2008).
If say Microsoft have incorporated Silverlight with extended mouse functionality, and the developers who doesn't understand this functionality and/or its limits went on to implement such behaviour with some serious business logic, the end user accessing the application who can't access such behaviour due to hardware limitations can't access such business logic, rendering the application useless.
To wrap it all up, Silverlight doesn't have extended mouse functionality for the same reasons as Adobe Flash doesn't.