I have two alternative suggestions:

1. As stability of an NPC decreases, so should its attack and/or defense. (Non-alertable only)

2. Make stability a simple measurement of total hull+shields remaining. (As it currently is for alertable NPCs)

As it stands, stability seems to be an arbitrary measurement which is not hugely useful for the majority of non-alertable NPCs.

Actually, if this did happen, I think it should be a function of Hull only. Not Hull plus Shields. The ship would not take any real damage until the shields were down anyway, theoretically allowing all systems to function at full capacity until the ship starts taking real damage.

I think NPC's are good as they are though.

Aside from elites and bosses, the status of an NPC in general has no real meaning. Some ships will be at 100% until the last tiny bit of damage, at which point the difference between 3% and 93% is almost nothing.