I know what you mean but since I use classes I have every character with a role that is useful to the battle, I don't need to have the same Materia combinations on the characters to be effective... Any class could have its usefulness even against Sephiroth...
Of course losing the White Mage (Aeris) on the course it helps that any of the 3 parties includes someone able to heal, although the Item command gives some self-sufficiency to everyone...
Having lost Aeris the most effective healers could be Cid with some of his blue magics or Vincent with his chemist command (W-Item)...
Yes, you got it! When Aeris is gone, no more healing spells! Losing a character means also losing his/her role in battle...
Too easy to simply switch her Materia to someone else... A loss is a loss...
Also, as you already noticed... She is the White Mage/Summoner... So no Knights of the Round... Again, too easy... She is the magically powerful last Ancient alive, with her Summoner class brought to full potential (Knights of the Round) she would have been the key to defeat Sephiroth but he kills her before she could achieve that and the party has to renounce to the easy way around...