The Sycamore Leaf archivement kinda of..

[Yoohre_WildRiver]

Using that slot for another accessory to convert will get you more desirable materia faster. The SB+ items are kinda bad :/

PS: All materia increases spiritbond rate for the item its bonded to at a MUCH higher rate than the SB+ items. eg a hat with a fire 1 materia melded onto it will bond noticeably faster than a hat with nothing melded.

so materia has a secret + SB stat in it??

[Zetonegi]

Only for the item its melded to but there's enough 'useless' materia that it isn't much of an issue.