No matter how you look at it, armor technically equals hull. Shields != hull: shields recharge, while hull takes permanent damage - armor mitigates that but doesn't recharge, making there be no "flee from battle to recharge armor" tactics. For example, let's look at this fighter:

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FIGHTER
- SHIELDS: 50 P
- ARMOR: 50 P
- HULL: 50 P

If 50 P of armor equals a 50% reduction in damage, then it will take:
- 50 points of damage to destroy the shields
- 100 points of damage to destroy the hull, as the armor repels/absorbs half the damage.
In other words, the fighter might as well be this:

FIGHTER
- SHIELDS: 50 P
- HULL: 100 P

Or, 15 shots with ten points of damage each.

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This time let's pretend that armor can be whittled away slowly over time - let's pretend that armor takes 50% of the damage it receives regardless of how strong it is, and the percentage of armor left determines how little damage the hull takes. We'll use the same fighter as in example 1: 50 P in each shields, hull, armor.

The shields take five shots @10P to destroy. After that:
That's eight shots @10P each, giving the fighter a rough equivalent of:

FIGHTER
- SHIELDS: 50
- HULL: 80

That means the ship was destroyed faster with armor than it would've been if it just spent money on extra hull, so unless your ship is absolutely packed with armor, why buy armor to begin with?

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If we assume armor is just an additional layer of protection before your ship's systems start failing, then you have something with a little gameplay value, but still not much; the fighter still basically has 100 P of hull - the systems just start failing after you hit half-health.

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If we assume that armor is weaker to a different damage type than shields, and hull is weak to everything, then things get very slightly more interesting, but hardly so as - why would you want to destroy just shields, or just armor? At best you're wasting an approximate equivalent on one as you're gaining on the other - it's still 15 shots.

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The only way armor works well in gameplay (enough to justify its existence) is if armor subtracts from the amount of damage your weapons do; in other words, you can only deal damage to a ship's hull if you have, bare minimum, 50P of damage with each weapon shot. Unfortunately, this means that smaller ships will be utterly incapable of dealing any damage to you at all, which makes armor very, very OP:

FIGHTER
- SHIELDS: 50
- HULL: <infinite>

HOWEVER... if we additionally assume that armor is chipped away by 50% of incoming weapon fire, then that hull can suddenly be attacked:
Although that's still the equivalent of a 150 P hull, you have a tiny bit of added gameplay value there - much like the shields, your hull doesn't take damage until the armor is damaged enough... but that still makes it feel very unbalanced and overpowered because the best strategy is to dump as much cash into armor as possible - and it doesn't even add much to the game. It's just an additional layer of non-recharging shields.