I don't know if anyone has suggested this already, but since we had some new minions at Dara's recently, I was thinking that a lot of people (including myself, I'll admit) often wait for the minions from there to go down in price and then buy them from usershops because we don't often have the patience to wait to visit them every day.
However, if there was a tiered achievement for bonding with minions, we might feel more inclined to make use of that feature, especially if there's a prize of some kind to work towards, like a forum title or border. There could also be some achievements for bonding with specific minions, although those don't have to have prizes attached, they could just be there as a completionist thing.