I don't think that side armor does require the relative position of weapon and target. One can simply separate the hit box in 3 parts, front mid back.
Angle of attack is separate feature.
It does, when the only factor that matters right now, is the location at which the shell impacts on the hitbox. More so when we had 2 factors into account, scatter and accuracy shots been (probably) centered towards the middle of the target.
The benefit of the current system is that its simple and straightforward to implement and understand.
The issues which arise are that even with perfect positioning, you can have unwanted results due to scatter, elevation and hitboxes.
The point of adding side armor should be to further improve the game and reward players for better positioning. In the current system, it would add a further layer which would indeed incentivase flanks but would also randomly punish players even more.
I should clarify, that my vision on angling for CoH, would be simple to determine what type of armor value (front, side, rear) should the shell use if it hits a vehicle disregarding which part it actually hits. It would roughly be a 120° split for each one (probably a bit more favoured towards the front than the sides).