Ditto is an excellent revenge killer, no doubt about that, but he needn't be so one-dimensional. If you're ballsy, he can function as a check to a lot of big threats thanks to the 'Trace-like' qualities of Eccentric. You can come in on the likes of Heatran, Jolteon, Flygon, Vaporeon etc and use their own ability to block their primary attack. You can come in on Pokemon with Intimidate and cut their Attack, or get in on a SubSeeding Erufuun and Taunt/Encore it to force it out. You can trap Magnezone/Dugtrio and give them a piece of their own medicine. If you mispredict and get paralysed or burned, you can switch in on Blissey/Celebi, acquire Natural Cure, and switch back out. There are hundreds of different uses for Eccentric. Revenge killing is likely the best use, but don't limit yourself to just that.