Yeah, this might belong to the Video Game Franchise forum. Or the VPFC, but I would say to ditto that it is somewhat useful to have it. It can morph into the opponent and can help you if one of its attacks is its weakness.
It depends entirely on your style. We are familiar with the breeding usages (that is somewhat creepy when you think about it but meh) and as for combat...I seen ditto used effectively before, and it depends on your style but....on the whole not necessarily the best choice.