Reks makes Vaan a bit more compelling cause the player understands his loss. I also loved how you learn more about Reks from the NPC's and certain side quests. Especially the "Jovy, The Hero" side quest.

Besides it's an unwritten rule in the FF universe that you are not allowed to have the "cool characters" be in your party. Golbez, the Dawn Warriors, General Leo, Zack, and Seifer...