We definitely don't.
The only reason we have 2 is because Alliances are all about pretend challenge. The mechanics are designed to bait and kill first timers, then you just scrape everyone off the floor with infinite Raises and beat it anyway first pull. This means you get your weekly chore out the way quickly and see the story, but also has the illusion that it was somewhat difficult on your first run because you died a lot.
It's poor design in my opinion, that leads to redundant safety comps like 2 healers, low damage intake and the whole thing becoming boring after a few runs when everyone knows the trick to each boss. But since ff14 players are allergic to wipes or having to play even moderately well, here we are with one player in the group simply existing as an occasional "just incase" in the background and everyone being fine with that.