It seems that the team responsible for Astoria have decided to go for the traditional Microsoft “embrace and extend” approach to the beautifully simple world of REST.
The whole point of REST is that a GET is a GET and a PUT is a PUT. GET reads a resource, PUT updates it. These operations are idempotent and simple to implement both for the server and for the client. Any requests which non-idempotently change state should use POST.
It seems that, rather than put some thought into how to expose resources in a way which allows a combination of these basic operations to perform any needed task, the Astoria team have decided that new HTTP verbs are required, and are now championing the addition of a MERGE verb.
This is (in my opinion, of course) flatly wrong.
Conceptually MERGE is a loosely defined operation, which will by its nature be implemented differently in different contexts, so nether the client software nor the network infrastructure will be able to make any simplifying assumptions. Worse, if it comes into any kind of common usage it will effectively require all servers and clients to be aware of it.
This verb is simply not needed. If the resources being addressed are too big for a GET/modify/PUT cycle on a whole resource to be effective, just provide a way of addressing parts of the resource.