I think its all about how much STR you really need..
From my experience 15 STR with a normal katana is enough to 2 shot most players. With a masterwork katana 12 STR is enough.
Theres no point in taking 95% of someones life out with the first hit. you still need 2 hits :-)
Of course all this depends on the enemy armor and IF, but I'm talking average usefulness.
My last gen had 12 STR, and no throwing, no IF. You are made of paper with little room for mistake, but that's how I like it. The 9 athletics give a lot of room for dancing around, choosing your battles, and being where you are "not supposed to be yet" in the battle. Also, dodging projectiles becomes much easier, archers miss you because they lead their shots behind you etc etc.. Basically: Freedom to control your situation in battle.
You also safely know that almost no-one can outspam you, and you can force 1vs1 situations even fighting multiple opponents.
Now I'm going with a 15/21 build with 5 pthrow, and already I have much more "stand and fight" situations, dodging is much harder etc etc..
Of course, you might want 15 str to use a better sword than the katana..
In the end the High Agi build lends itself best to light armor as they are beneficial to each other, both working towards speed.
If you plan on using heavier armor, IF pays off much more as armor makes those HP last longer.
Heavy armor is friends with STR and IF (To a point. More than 18-21 str is wasted as you 2 shot almost everyone anyway)
Light armor is friends with AGI (To the point where you need 3-4 strikes to kill anyone, getting glances etc..)
PS: Also i might be jaded, as I always always go for headshots in battle.. All this depends on your skill, your ping, if you have an accurate mouse etc.