It was allowed in 3.5; it should be allowed here.
Melee still needs more buffing and this is one way to do it. Clerics and FvS are not going to be able to take both without some sacrifice to their builds. It opens the path to Whirlwind attack for the Fighter and Tempest by not requiring them to take a worthless feat. It buffs melee against melee attacks which is how it should be. It gives an incentive for Melee classes not to multi-class because they will actually need their base to-hit.
Someone made a mistake at the beginning of the game to not allow this and it has become tradition. This tradition should change. It won't ruin game balance and will add more variety to the game.
At the moment , the only players taking expertise are sword and board intimitanks. Allowing them to use power attack is not going to break the game.