I'm not actually WG, and I hold no weight at all, but from experience in clans, and now with Ronin, the 80 def. requirement is an excellent, excellent one to have. It helps avoid constant KO's from people, because sometimes, in the bigger fights, even you are a great tank, and can get around objects, with lower then 80 def, your never going to be able to even show it. It's tough to get for some people, but it really is a necessity to have to be successful.
Edit: and there actually is a pretty big difference, every 1 level is a big add on to your last level, and then you say, 75 is not much of a different from 80, and then people will say, why not 70, it's not much of a difference from 75, then people will think, why even have it in the first place?
Anyways, it encourages people to train Def, which is really the best cmb skill to train up anyways.
~Po