Well, yeah, but then he could grow real big and smash things - he didn't NEED any fighting experience beyond what he already had, which was not inconsiderable. As Ronin, he's basically billing himself as a modern-day samurai, which kinda comes out of nowhere - at least with the whole Goliath thing, he'd been around Giant-Man before and had a chance to think 'heyyyy - that whole 'growing big' thing looks like a real gas! If the arrow-slinging game ever gets old, maybe I should try that!' But martial artists have never featured heavily on the Avengers' roster.