What EXACTLY is the advantage of 100% spiritbonded item?

[Renius]

Other than to break it down later and hope for a materia of some relevance to what you may be able to use.

[ManuelBravo]

De synthesize it for other types of materia and more materials. You can sell these or used for other advance crafting.

[Niwashi]

Desynth doesn't require 100% spiritbond (or any spiritbond, for that matter). The only advantage 100% spiritbond gives is the ability to convert the gear to materia. (And that's provided it's a type of gear that can be converted in the first place. Non-convertible gear gets no advantage from spiritbond.)

As to hoping the type of materia is what you want, while that's of course preferable, there may be other people who want some of the types you don't, so you could sell it. Or if you want to take another spin at the RNG chances, you can trade several materia you don't want to Mutamix for a new one (that may or may not be more useful than the ones you traded in for it).

In the worst case, the most useless materia you could get for a piece of gear will generally still be worth more than the pittance an NPC vendor would give you for selling the used gear to them, and in many cases, those are your only two options for getting something for the gear when you're done with it, if it's not a type of gear that can be turned in to your GC and you can't desynth it (because you don't craft, don't have dysnth skill at the right level, or it isn't from one of the 3 classes you're doing desynth for).

And in the best case, you could get one of the really useful types of materia.

[ManuelBravo]

The higher the spirit bound the higher the number of materials you can get as well as skill playing a mayor part. When I started I gotmore materials out of a 100% then a lower percentage.

[Niwashi]

The higher the spirit bound the higher the number of materials you can get

Ah, I didn't know that part. I knew it didn't affect success/failure rates, but I didn't realize the amount of items was also variable (or rather, that there were factors other than just the RNG controlling that variability).

[KarstenS]

Other than to break it down later and hope for a materia of some relevance to what you may be able to use.

There was something planned during the closed beta. Something like "they will get to ther full strengt when bound". But they gave up that idea. So its only use is for converting into materia.