There are quite a few reasons I'd like to see this happen.

1. UVs go up to +4. That's 2/3s the max bonus for ASI and 100% of the bonus needed for CTR. With this UVs have less of an impact because they're either +2/5s the max ASI buff or +1/2 the max CTR buff. This also effects the bonuses gained from things like black kat and chaos.

2. It would allow monster bonuses from things like wild hunting blade and more importantly damage VS X UVs/Sprite perks to stack with armor like black kat and chaos.

3. It allows for more variance in armor and more creative armor solutions for balance. Lets say you wanted ancient to function-

New Ancient:
+6 (instead of +3) health
+5 damage up per piece
-2 ASI per piece
-2 MSI per piece

This would give it a total of +10 damage ups which would be pretty swell but at the cost of some asi and msi, which would limit your overall DPS. This would benefit you if you can plan your attacks wisely, as individual hits would be quite powerful. Think of unleashing a slowly charged blitz needle shot into Vanaduke- you're dodging most of the time anyway, so this play style benefits over the constant damage output of CTR + damage buffs that currently reign supreme.

4. Since you can have more stats you can give more stats out easily which plays well with CTR especially. Cobalt? +1 global CTR, why not. Skelly? Stick a +1 damage onto it. This kind of change makes it easier for knights to get to decent stats with a wide variety of options.. especially when it comes to CTR- a +2 bomb ctr would instantly open up bombs as a viable weapon type on a lot of different armor sets, and give them a way to function.

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What this means:

Damage.. actually not much. It just gets about 16-20% better based on weapon. But you have to actually get to that level of damage, so...

ASI... would allow the knight to attack ridiculously fast. This might be good balance wise for some weapons, like blitz needle, heavy swords, things like that. There would exist a sweet spot at +10 asi and +10 damage where the knight would be doing an insane amount of DPS with certain weapons, but is that really a bad thing if the knight somehow managed to get to +10 damage/asi? Lets say we went full black kat, swiftstrike, trinkets...

Black kat = +6 damage
Swiftstrike = +3 asi
Trinkets = +4 damage for that weapon type
Sprite = +2 asi for that weapon type
UVs = +4 asi

...so the most you could really get using popular damage bonuses would be +9 asi and +10 damage. This woulid be amazing but you'd need a bit more to max out.. which would force the player to use damage vs X buffs over weapon trinkets- forcing some semblance of balance.

CTR.. would need changing flat out with this but it already doesn't function very well from a balance perspective. If each level of CTR did less this would be perfect- bomber armor could be bumped up stat-wise-

2* = 1 ctr
3* = 2ctr
4* = 3 ctr
5* = 4 ctr (volcanic demo)

This way you'd end up with even faster bombs than before and still max out. Meanwhile chaos isn't as effective and mad bomber can be given that +4 it deserves. See how easy that balance is?

Black Kat =
+3 damage
+2 MSI
+0 freeze resist

Chaos =
+3 damage
+2 ctr for all = 6/10, so you'd still be able to use a bit more.

Mad bomber =
+5 bomb damage
+4 bomb ctr
+1 MSI

MSI... would also be effected and could be used to balance out new armors, should that be an option.. but it's already capped at +4 so I don't see it.