Versatile is for small sized characters, basically. Not defending it, but that's basically what it's for. A halfling can use a longsword in both hands to get a d10 without disadvantage.
And I completely agree, there are many design choices in 5e that I can't get my head around, since they don't seem to make a lot of sense. The entire design around saving throws seems backwards to me. I would have preferred 4e's approach, or something similar, which would let you use one of two ability scores to determine a save/defense (so Reflex is based on either Dex or Int) and have three of them, which is super elegant.
Plus, the game isn't clear exactly what makes a spell need a Charisma save, either. I mean, you look at Banishment. Why is it a Charisma save? Because nothing else makes sense?