I'm not sure where I stand on the under 23 issue.
I do think that it allows athletes more championship experience allowing them to perform when they make senior championships - see Rimmer, Ellis, Lyne, Farah for example. Three medalists in their first senior competitions. It is clear that they have benefitted from their u20 / u23 championship experiences.
I also think it gives 20 / 21 yr olds somthing to aim for whilst they are still developing physically into adults is a good step. Not all athletes are capable of putting in the miles necessary to be a top distance athlete at 20.
Something Nic Bideau picked up on a few months ago was that he put his athletes into races where they can win, rather than flogging them against competition they cannot cope with. I suppose an u23 championship will allow messer's Vernon, Hickey (sorry don't really know all the names) to compete on a level where they can be competitive rather than putting them up against athletes of the likes of Lebid, De le Ossa (sp?).
I am slightly concerned that it maybe another championship that doesn't really hold too much prestige may be saturising competition and adds to the ever increasing length / size of the XC season.