I think that her spirit should have forged with materia. When the materia fell out of her hair Cloud should have had to swim down and retrieve the materia (from an angry Sahagin boss! ). When you get the Materia you find it's useless...but level it to max and you unlock a new summon...Aeris. When you Summon her she casts barrier and restores your health so even when she's not around she's always there watching out for her friends