I find it amusing so I keep using it, but I've seen no real incentive to convince me its actually necessary. There's lots of overworld stuff to do though, so I wouldn't worry too much about mission experience. You can take whoever you want into overowrld fights, and there's a mechanic where you can grow challenge and experience by accepting enemy back-up fights which can get insanely difficult for early levels. They start out a little lower then you and then gain like 5 levels for every fight or something. I made it to fighting level 30 enemies when my people were around 12 and almost got wiped because I'm still getting used to combat mechanics for some reason and my AI team was eating rocket launchers left and right and going out in one hit. I almost didn't make it. But it alludes to an interesting risk/reward mechanic that can make a random encounter a little bit more interesting