Yes. In other words, any HTTP request message is allowed to contain
a message body, and thus must parse messages with that in mind.
Server semantics for GET, however, are restricted such that a body,
if any, has no semantic meaning to the request. The requirements
on parsing are separate from the requirements on method semantics.

So, yes, you can send a body with GET, and no, it is never useful
to do so.

This is part of the layered design of HTTP/1.1 that will become
clear again once the spec is partitioned (work in progress).

....Roy