Personally I think we should all have each other's backs as cyclists on the road whether roadies, mtb, duchesses etc. In effect someone slower 'barging' in front isn't too dissimilar to a cyclist filtering in front of a car or motorbike- ok we have the ASL on many occasions and are totally entitled to use it but I'd imagine motor vehicles often think the same.
Saying this, I never cut in front of another cyclist when stopped at lights (unless there's masses of room in the ASL and a clear slot). Get plenty of people doing it to me, but I really don't care. As soon as traffic's moving and the coast is clear they're usually not in front for long ;-) Maybe it's the case that some cyclists feel the need to cut to the front as they feel uncomfortable in a pack. I've actually found older gents (40+ - older than me anyway) to be a bit bargy. Have had to give an 'easy Boris' when a bojo's almost wobbled into me on a couple of occasions!
As I've said in a few posts, commuting just requires a bit of patience and composure, even if you're a speedy gonzalaz riding a scalpotron 5000. The bloke in the OP sounds a bit of a douche, but hey, you get 'em.