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Personally I think mega pokes like Sableye and banette get a bad rap because players haven't figured out how to really use them yet. All of the really popular megas at the moment are basically JUST mega pokemon - they are intended to mega evolve immediately and simply BE the mega evolution. Few people have tried using them modularly.
Mega Sableye would be a modular mega pokemon. Honestly you probably want it to just be regular sableye most of the time, filling the old niches, but the megastone would come in handy in particular situations where the opponent's team calls for that utility of having a wallish anti-lead or anti-setup option. Mega Sableye is NOT intended to be mega all the time. Maybe not even most of the time.