Even if they're at max level they count as one of the members when dividing the exp at the end of a battle. So even with Cloud maxed your exp will still be divided by 5 for a full party.
As for the Aerith gatcha, the Staff is a good black mage weapon and her ribbon is very strong SB. The three bangles are all decent as well what with having ~75 def/res and 20 atk/mag/mnd. I probably won't use 50 mythril on this but I'll do the 100 gem pull and maybe a 3 pull as well. I'm surprised we're fighting the materia keeper, lost number and Jenova Birth, AGAIN. Here's hoping they throw in Red XIII's crystal too. That would be pretty sweet, I could replace Wakka with him since I have his Protect/Haste SB.