I'm pretty sure a 72 year old was not a worse liability to the stealthy commando team than a shiny golden who doesn't shut up or the principal cast which has no real reason to be there other than they happen to be the heroes. I mean Han and Chewie, sure, they did missions like that all the time but Luke should have been in an X-wing and Leia should have been anywhere else, because do we see anybody like Madine or Mon Mothma during the battle on Home One? Nope, they wouldn't be any good during the battle.
Anyway, I'm all for Rex being Willie Nelson: Rebel Commando