I don't even think a flanking bonus is necessary, but at least that's a rule in 5e and makes sense.
They have to include either backstabbing or flanking with the way the game incorporates verticality. Otherwise, if ranged characters get advantage for elevation but melee characters get no advantage, then melee characters will become incredibly underpowered.
They could remove both backstabbing and advantage from elevation...but tbh I like height adv/disadvantage. It's somewhat realistic and keeps things fun by preventing too many misses.
