Emperor why the hell would you ever run Leftovers? Speed tying will just get you swept half the time. Choice Scarf lets you kill, and sometimes sweep.
Well, you never know.Leftovers would let it counter different Pokemon...Say Water Absorb Calm Mind Suicune.If it only has surf, then you could waste all it's PP.
I've been trying to figure out another way to use Ditto.I came up with these:
-Spinner/Spiker....Come into a Spiker/Spinner and set up your own hazards or just spin away.This will work because most spinners/spikers tend to focus on defense, meaning they can't really hurt themselves(example:Forretress,that Grass/Steel guy would need to carry a fire attack..not likely.Tentacruel also cant really hurt himself, and that Sp def helps..Skarmory).Though you might say his HP is too low, however like i said, you shouldn't be taking strong attacks anyways.Also note that some of these guys have HP around his.
This Ditto would switch into the opponents supporters and do what it was meant to do since Gen 1...Use the opponents strategy against them.
However,what if your opponent doesn't have these types of Pokemon in their team?Is Ditto a waste of space?No more than any other revenge killer that ends up going against a bunch of slow pokemon/stall.And even then, you can always fake a scarf with other items.
If however your opponent does have a stallish team, Ditto would be a spinner/spiker all in one Pokemon.
Notice how if Ditto is going against stall, he will be able to heal himself with their own moves.
I also see this other use for ditto:
-Lead with Focus Sash.
This will let you see their moves and strategy.If all the moves are pure attacks, than hes choiced.Focus Sash will also allow you 1 turn of setting up stealh rock/etc.If say, they are trying to set up something else that is not favorable, you attack.Pretty much any lead will get screwed up by this,since you got your sash..