WebGL does not currently have a large reach: it is only on by default in Chrome and Firefox, and even then about half of that user base has out of date drivers so WebGL gets disabled. IE shows no signs of supporting it at all, but Chrome Frame will enable it (as long as the driver is not too old). Opera supports it, but it's off by default and must be switched on. Safari for Mac supports it but only on Mac (not the Windows version) and again off by default. Currently we've disabled WebGL on all mobile devices because it's either broken or a lot slower, so no shaders on mobile for the time being.
However, it will improve: I believe Google will add WebGL to Chrome for Android, Firefox Mobile already supports WebGL (although it needs some work to be faster and some bugs to be fixed), Opera and Safari should eventually switch WebGL on by default, and as people gradually replace their computers the number of people with WebGL disabled due to old drivers will gradually fall. IE is also still losing market share to Chrome at a steady pace.
The main reason we're adding WebGL shaders now is because of the EXE exporter. With Awesomium, WebGL is forced on for all computers. This means if you export your game as a desktop app, you can guarantee shaders will be available.