Yeah, everyone not in your active party gets 1/2 the EXP your main party gets. Even if they're KO'd.

Ontopic, I always used Aerith on my latest few playthroughs simply BECAUSE I know she's going to die and I want to wreck some trout with her before she does. Say what you will about the usefulness of her Limit Breaks, watching Demon's Gate do 0 damage to your entire party is pure delight. Plus, you know, achievement.

Admittedly, she is no powerhouse and her low Strength stat somewhat mitigates her high Attack Power with the Umbrella (plus, Red's Seraph Comb is better), but it's your only chance to use her, so might as well.