After having worked with ExtJS/Webnodes for a while now, I'm not a 100% dependent upon MVC support, but almost every ASP.NET/Ext JS developer out there is recommending MVC and not Webforms in combination with Ext and similar JavaScript frameworks.
The big upside is cleaner application structure, cleaner code and guaranteed increased productivity once the developer is up and running in the technology.
On general basis I want to use MVC no matter what, because it gets me back to "basics", leaving all the desktop-stuff behind which Webforms was designed to deliver for web. It forces me to separate concerns, it's more JavaScript-friendly and more testable.
Having said that, developing large applications with complex forms might be a bigger job in MVC than Webforms (I'm not sure), so it might be a good idea to consider each option for what kind of project you're going into. As with all technologies there are always upsides and downsides...
But the whole idea of switching to MVC entirely is very tempting! :)