You shouldn't have to wear gear that you're attaching to. Here's why:
1. Matsui's new battle system supposedly focuses on wholehearted play, where effort is rewarded and halfhearted devotion struggles.
2. Attachment points are a grind-based system, where many hours of activity goes toward gaining attachment. Thus, you will spend a lot of time with that gear you aim to turn to materia.
3. Gear that will be turned into materia will economically be the cheapest piece that will result in the desired materia.
4. Combine points two and three, and you have people wearing the cheapest pieces for long amounts of time.
5. Point 4 sets up a system where players are encouraged to wear lower quality gear than they actually own, to benefit from materia grinding.
6. Or, alternatively skilling up materia on their own time in grind solofests, which is boring gameplay.
7. If players bring cheap gear out to performance venues to get materia on, like SP parties and raids, that lack of gear quality inherent to the materia grind undermines Matsui's goal.
8. At that point, you have people not putting their best foot forward on purpose just to achieve a gameplay mechanic goal. This is counter to the spirit of Final Fantasy. It is not "Final Fantasy" to not put your best foot forward on purpose, nor is it final fantasy to whack moles for hours just to wear down gear on purpose so you can convert it.
As such, it is better for attachment of gear to be independent of you equipping it, that you can choose ANY gear to attach to.
Four benefits you can't get otherwise.
1. Players have every reason to put their best foot forward in battle, at all times.
2. Players can still grow materia through activities at the peak of their performance abilities.
3. Players will not need to grind materia on their own time in meaningless time-extending gameplay.
4. This method eliminates potential conflict between players who would rather perform at their best than get materia and those who would rather perform less than their best, but with the ability to get materia.
With this system, it simply becomes everyone performing their best, and everyone getting materia.
There are no downsides. There are only positives. You should not have to equip gear to materia it.
The gut reaction is to balk and say of course you should have to equip gear to attach to it. RESIST that urge and critically think about what that makes players do, for no real reason.