I don't know why you wouldn't; it's like free experience points per step! The only drawbacks are lack of EVs, the cost of retrieving the Pokemon from daycare custody, and the potential that your moveset might get screwed up.
I tend to use the daycare for Pokemon that I might use occasionally but am not focusing on them much at the moment... When I start my "actual/official" files, this will be the perfect spot for all of my Unown and Ditto post-EV training, since they only have a single move in their movesets and the fear of a screwed-up moveset is effectively nullified.
And honestly, manually training a Unown would get really boring, not to mention grueling and tiresome...the Elite 4 and Champion are already monotonous enough in their rematches! Ditto less so, because of the move Transform's unique, quirky nature.