The thing is, against at +2 / +2 Haxorus (or anything for that matter) you don't want to lose the Speed tie. If you do, you're screwed. Furthermore, it has a measly 48 base HP, and 5 PP per move, meaning even if you copy a bulky sweeper to set up and countersweep, you won't actually be that bulky. Also, when you bring it in against a Choice locked Pokemon, for one if the opponent is locked into a move that hits itself super effectively, what's to stop it from switching out; and second, if they stay in and you lose the Speed tie, Ditto will still be down HP, of which it didn't have a lot in the first place.
IMO there are too few events that let Ditto perform well without Choice Scarf that you might as well slap one on and act as a mostly catch-all revenge killer.
That being said, I have dabbled with Red Card Ditto a bit, where if you lose the Speed tie you aren't completely screwed. However, you have to survive for it to activate, and that's not every easy unless you're facing a bulk up or Calm Mind user.
[21:11] <&Oglemi> opera da best
[21:12] <%Birkal> I love opera
[21:12] <%Birkal> our college put on cosi last year; it was great
[21:12] <%Birkal> OH MY GOODNESS
[21:12] <%Birkal> THIS CONVERSATION IS ABOUT WEB BROWSERS
[21:12] <%Birkal> =X