ASP.NET was originally mean to be deployed as a single piece as part of the .NET framework. However, since they are branching out to different stacks such as MVC, I think you will eventually see Web Forms seperated from System.Web and deployed as an add-on stack just like MVC.
Razor is independant from MVC and WebMatrix. Razor is the name of the markup that both of them can use. It's just text, you don't need anything special, and you can write it in notepad if you want.
There shouldn't be any worry about "extra add-ons" since MVC can be deployed as a self contained unit. Just make sure the server has .NET4 and use "Add Deployable Dependencies" and MVC will work just fine.
Recommended reading: http://www.asp.net/mvc