"Transform changes the user's current type, current stats, current stat levels, current moves, current species, current cry, and current catch rate to that of the target's."
Question . . or several:
Won't Ditto need to be EV trained and have the appropriate nature for competitive play? Using DemonPikachu's example, if the Ditto were Impish and transformed itself into a Salamence of Adamant nature, will it hit Salamence hard enough and fast enough even with the copied stat changes, seeing as Transform doesn't allow it to copy the Pokemon nature? The nature itself plays a role in boosting stats and EV training is to enhance those stats ideally.
If the other player switches their pokemon, will DW Ditto automatically transform again to the new pokemon on gameplay? If the nature isn't copied, how do you know that Ditto will be effective in this scenario as well?
If you're gearing Ditto to battle a particular pokemon say (i.e. using Salamence again), how can you be sure that that Pokemon will appear on the opposing team? It may be OU, but not everyone will use it, and you don't know what the opposing team will have until you confirm the battle via wi-fi connection.
Is it really true that Ditto's DW ability can't be passed on via breeding? If so, people searching for the right nature will have a hard time hunting for the right Ditto in DW, just a minor issue.