Best duel of the DM series IMO, just two complete chads boxing the shit out of each other with an awesome card type idea.
Can't even fault them for doing it over a woman because Mai is hot, even if she's a cheap shotting bitch with no gratitude.
As for Valon not nuking Jinzo earlier. My best explanation for it, is that because his philosophy is "all attack" he didn't want to effectively win due to a defensive play. Also he's trying to determine what Mai sees in Jonouchi so he's dragging it out.
Too bad Mai decides to be a coward and challenge Jonouchi IMMEDIATELY after this, when the guy can barely stand upright, much less duel. How the hell do you "prove you're better" when you only won because your opponent was battered and exhausted?