I have thought about it some time today, and I think there are 2 problems with units having a defence bonus before their first attack:
It makes approaching archers too easy, as they lose their advantages of shooting first. It could be rebalanced by giving archers more attack power, but then, it will create many other problems, as they will destroy units already engaged in melee (while it should be much harder to hit someone already in melee in case you mind spraying your friends with bowfire).
To compensate, it would now be possible to use meatshields that do not attack at all to protect your archers to keep their defensive bonus while the archers slaughter the opponents, which would be weird.
I think a better fix would be to decrease an unit accuracy if it has moved before, that would solve the kiting problem too (archers running back and shooting to get 1 more shot before melee).