Unchained Automatic Weapon Progression, how exactly do Magic Items work with Atunement?

[Aldath]

"In this system, magic weapons, armor, and shields never have enhancement bonuses of their own; those bonuses are granted only through attunement. Any weapon, armor, or shield special abilities on attuned items count against a character's enhancement bonus from attunement. To determine an attuned magic item's enhancement bonus, subtract the cost of its special ability from the enhancement bonus granted by attunement. (This applies only to special abilities whose cost is equivalent to an enhancement bonus, not to those that cost a flat amount of gold pieces.) For example, if a character with a +3 enhancement bonus from weapon attunement wields a keen scimitar, she subtracts 1 point of her enhancement bonus (for the cost of keen), leaving her with a +2 keen scimitar. If a character doesn't have enough of an enhancement bonus to afford the special ability (such as a 4th-level character with a vorpal longsword), she can still use the weapon's power on its own, but the weapon gains no enhancement bonus."

This paragraph lost me...

So, a character with a +5 attunement bonus using a Vorpal Blade would have 0 attack bonus anyways? I mean, if I have to subtract the cost of the enchantment from the attunement, It would be 0.

Doesn't this makes magic items significantly weaker? Or am I missing something?

[Zahir ibn Mahmoud ibn Jothan]

I think you're missing nothing, and this part of the Automatic Bonus Progression system is broken.

Why would you attune it under those circumstances?

Why wouldn't you just have a weapon be your +5 weapon, and then use your vorpal weapon unattuned as...

PF Unchained wrote:

she can still use the weapon’s power on its own, but the weapon gains no enhancement bonus.

[Calth]

This blog post has an update to the automatic progression system that is a lot better than the one presented in the book.