You should be able to stack another type of protection (projectile, fire, or blast) on to normal protection. Currently, other forms of protection besides the normal one are useless, but this would give them a use without it being overpowered.
The precise reason they aren't compatible is for balance and to encourage choice.
The standard Protection enchantment gives additional defense against pretty much everything, whereas the specific protection type enchantments give even more defense than that, but only against certain things.
Also, I think the specific protection enchantments (Blast Protection, Fire Protection, and projectile Protection) would allow you to have an excess (overcap) amount on their own, let alone if stacked with the standard Protection enchantment. This is because, again, the values are balanced around being able to exchange some standard Protection out for specific types. In other words, the values would need adjusted if the system was changed, and all you accomplish at that point is making it mandatory to have maximum protection of all types versus just one right now. This adds grind and removes choice.
Blast Protection is good.
Fire Protection itself isn't terrible, but other things in the game (healing from full saturation, and fire resistance potions) render it unnecessary.
Projectile Protection is pretty useless.
I'd rather not have to get more enchantments (which would also mean needing to rework the enchanting cost system) just to add extra enchantments that are, quite honestly, not that useful.
Make a second helmet with Blast Protection and a gold nether helmet with Fire Protection and you're fine (maybe swap the latter and/or make a third with projectile Protection if you're fighting a lot of skeletons in a soul sand valley?).
You should be able to stack another type of protection (projectile, fire, or blast) on to normal protection. Currently, other forms of protection besides the normal one are useless, but this would give them a use without it being overpowered.
The precise reason they aren't compatible is for balance and to encourage choice.
The standard Protection enchantment gives additional defense against pretty much everything, whereas the specific protection type enchantments give even more defense than that, but only against certain things.
Also, I think the specific protection enchantments (Blast Protection, Fire Protection, and projectile Protection) would allow you to have an excess (overcap) amount on their own, let alone if stacked with the standard Protection enchantment. This is because, again, the values are balanced around being able to exchange some standard Protection out for specific types. In other words, the values would need adjusted if the system was changed, and all you accomplish at that point is making it mandatory to have maximum protection of all types versus just one right now. This adds grind and removes choice.
Blast Protection is good.
Fire Protection itself isn't terrible, but other things in the game (healing from full saturation, and fire resistance potions) render it unnecessary.
Projectile Protection is pretty useless.
I'd rather not have to get more enchantments (which would also mean needing to rework the enchanting cost system) just to add extra enchantments that are, quite honestly, not that useful.
Make a second helmet with Blast Protection and a gold nether helmet with Fire Protection and you're fine (maybe swap the latter and/or make a third with projectile Protection if you're fighting a lot of skeletons in a soul sand valley?).