It increases his ult damage so it encourages the player to be efficient with the Ult, Q and basically everything in his kit. The most I have been able to do with animation cancelling is E auto auto W Q auto and that's 5 stacks, but I have 100k pts on Darius anyway so I doubt any random Darius can just hit 5 stacks just like that, it doesn't even work all the time. Not to mention that it gives the sense for juggernauts that "you probably shouldn't be in my face" so he NEEDS it to be dangerous if just attacked randomly or has no respect given to him so he takes advantage to get fed off of the enemy's ignorance of his actual kit. It really puts his punishment in spot because once he hits that AD buff, he will do a lot of damage and he has 2 slows so you get pretty much trashed for trying to do what you just did. In my eyes, it makes the Darius and the enemy figure out how to play around/with Darius. This is why Bronze players who tend to not learn at all, get dominated by Darius.
However, this "hefty reward for doing something he is supposed to" get 5 stacks? I mean, that's not always your goal as Darius. You usually wanna last hit an enemy which can pretty much ramp up your power insanely and cause havoc in the team fight, and not to mention it's pretty novice of you to say "5 stacks" instead of "Noxian Might" because both are a bit different. If they were the same, he'd get Noxian Might just for attacking a cannon minion 5 times, in which he doesn't. 5 stacks to him is only a bit more important than Xin's need for the 3 hit passive. Noxian Might, is a whole other story.
Thats all I got.
edit: 1 of the slows is combined with the CC so you need to not make it seem like he is part
with a ton of cc