> "But it looks just like the old thing using <insert data scheme> and some templates?"
> "Ah yes, but now we're doing it in XML!"
I think this applies to 90% of instances where XML was adopted, especially within the "enterprise IT industry". Through marketing or misunderstanding, XML was presumed to be the "magic fairy dust" that would solve countless problems simply by "switching to it". The library world is certainly not unique in this respect.
Returning to the original question, "what is MARCXML for", I think there have been some very clear examples of where it can be useful to some people, sometimes. If it works for you, use it. It not, don't.
To wit, I propose:
"Some people, when confronted with a problem, think "I know, I'll use MARCXML." Now they have three problems: MARC, XML, and the one they started with."