That's a smart question. Thanks for asking it instead of immediately attacking the guy because you didn't share his opinion. So let me answer it for you...
On paper it might look like you're giving both the player and AI the same checks and balances on their power. But in execution you're honestly just hurting the AI more than anything.
The player is going to be smart enough to do these things:
1. Own every possible holding in a capital (even the temples and cities) then throw his marshal in there to train troops and buff his personal demesne as high as possible.
2. Make sure his capital is on a rich 6-7 holding county, instead of whatever the 'official' capital of the empire/kingdom is. Which is more commonly a 3 holding county.
3. Utilize the fullest extent of their demesne cap. A player mostly isn't going to be happy with anything less. Depending on how much his vassals like him, he may even push it further to 2 or 3 holdings above the cap.
4. Focus on militarization techs to buff his retinue ASAP. On top of making smart retinues instead of just picking stuff seemingly at random.
5. Boost their economy... by doing stuff such as use republic vassals, set tax laws high (AI usually prefers to keep them low), borrow+expelling the jews, asking from the pope, etc. All this helps you win the mercenary race. A player good enough at this can even keep mercenaries as a permanent standing army and still make profit.
6. Go out of their way to vassalize possible holy orders.
And so on... there's just so many things the player can do that the AI can't or won't. So with vassal levies at the way they are now you could argue that the game is actually easier.
So then it seems that punishing nations (aka players) that go over their limits is warranted. It helps correct an obvious exploit, and brings the situation at least a little bit back towards parity. The AI is never going to be better than the player, but at least we can make it offer a challenge.