Ditto should only be sent out against Pokemon that one wants to revenge kill
Not necessarily. For instance, if one is using Ditto against a team with say Forretress but no spinblocker, Ditto can switch in on Forry and Rapid Spin away all the hazards (JIbaku is yelling at me cuz it gets more hazards but I'm talking about something like getting rid of SR that a dialga set up earlier because Forry rarely carries all 3 formes of hazards by itself). It could also lay down some hazards of your own. You could also switch into Darkrai (after it has slept something) and use its Dark Void to put something on your opponent's team out of commission. It is mainly a revenge killer, but there's still some room for creativity!
Another important thing to note that when using Ditto you dont get the Soul Dew boost from Latios/Latias if you decide to switch into them. Also, your Judgment will always be Normal if you copy Arceus, unless for some reason you are using a plate on your Ditto o_O.