As you and everyone said...it should be kept as a treasure like Shanks had the Gommu- Gommu no mi, or how he treasures his hat.
However, were a member to get it.....hard to say....if it was a weapon that took it...probably Zoro's Sword or Ussop's Kabuto.
If it is eaten by the person...Nami
Simply because she's the one who needs it the most. Rescuing drowning DF. Users has always been Zoro, Sanji, or Ussop's job. Zoro wants to be the greatest swordsman so I can't see him taking one...would be like cheating. Sanji could have used it well, but his blue walk and sky walk would become useless. Also, the flanking diablo jambel leg he squirrel through years of working at a hot stove would become no longer needed as well. Just can't see Ussop eating it other than his weapon. He also already has fire attacks.
Nami I overall weak in combat unlss she has prep time. With the Merra- Merra she has options other than hoping she csn have enough time or that someone doesn't see through her Mirage Tempo. Also, she could use the fire with her cool balls more efficiently making a thunderstorm tempo!!!