Just for clarification since some people seem to be unclear, this issue is specifically about whether to allow/recommend HEAD requests when making the HTTP request to verify that a webmention source URL does in fact link to the target URL.
This is *not* about the HTTP request used to discover the webmention endpoint in the first place. There are definitely obvious benefits to doing a HEAD request first when discovering the endpoint, since the webmention endpoint may be advertised in the HTTP header, which means that the sender can avoid making a GET request at all.
When verifying the link back, the receiver is going to have to make a GET request eventually, in order to check the document body for the link. The question is whether we should recommend that a receiver first make a HEAD request to the source URL before making the GET request.