Even if they're max level there's still reasons to send them out, the weekly mission for the GC buff items obviously, but after that you can also send them out on other missions to get new chemistry. For example my squad has 20% chance to bring back gatherers scrips, 10% chance to bring back GC seals, 40% chance to find DoH materia and a squad member who increases the activation rate of other squad members chemistry by 20%. I'm currently sending out my other 4 members so they can build some good interlocking chemistries too.