Aww, give it some slack. It's a movie! For it to be as good as it is is already something we can appreciate and maybe even hope for in films to come (I mean, Kingdom of Heaven. Really??)
No need to always be so criticizing; it just makes us look mean!
The fight wasn't really that long; each exchange lasted no longer than Casper and Aaron's spar here:
http://www.youtube.com/watch?v=ZnqOMbFDEAI
Besides, the account of De Lalaing mentions him engaging in duels with other knights with poleaxes, whacking each other so much that sparks started flying everywhere with neither really getting seriously hurt, so it's not like weapon-on-plate didn't happen.
Other than head/exposed area strike attempts, whacking someone even on protected spots can help unbalance a foe that can lead to an easier takedown for a kill.
http://www.thearma.org/essays/Lalaing.htm
EDIT: Er... Don't whack plate armor with blades though...maybe with a half-sword+pommel or an armored knee like Aaron did in the video above...
"...But beware the Juggler, to whom the unseemliest losses are and who is found everywhere in the world, until all are put away." - Joachim Meyer