The kinda of people who benefit largely off wearing cross class accessories are the kinda of people who study (or at least read a little) about the full implications of wearing such gear. Start examining obvious end-game raiders (people who at the moment are wearing FCOB gear) and you will notice that for the most part the gear was created by the same person... which you could also assume they melded it as well... not just willy nilly putz'ing about on the MB and buying all the things cuz their rich... and then max melding them with vit I materia lol. But scaling the RIDICULOUS masterbook II wall and them pushing that gear to its highest level to reach caps.
Everything you said about these accessories (and even stuff about stats in general) is completely wrong. The people who equip this stuff know exactly which one they should be wearing how to maximize its full potential through melds. Even if you don't use it for that purpose, and want to use it how it was intended then its pretty clear that the tank should wear fending, blm caster, whm healing... And if you are to stupid to figure out anything i just said then you have no place at end game accessories and REALLY hope they dont start designing this game towards them.
edit: oh and making meld's even more of a gil sink (no 100% spot) would not stop people form doing exactly whats being done already... just piss em off because you dangled that spot in front of them then took it away.