the answer is: it depends.
you have to study the format where you want to play, but the main reason to use a ditto, according to me, is to "steal" the stat boosts from a specific Pokemon.
For example: in the past season of VGC there were lots of xerneas and it was used to set geomancy (duh) but with ditto you could enter in the field and copy the opposing xerneas, stat boosts included.
ditto is a cool niche Pokemon and used in the proper way to counter certain set up 'mon it could do very well.