Because since Ditto can breed with any Pokemon, it will make breeding 99.9% easier, but there has to be a negative effect.
Pleaselog inorregisterto add a comment.
No, there is no real downside to ditto breeding. You can not breed egg moves or anything like that(unless the non-ditto parent is male and has those moves) but for basic breeding there is no downside. Also Ditto can not breed with any Pokemon, it can not breed with Pokemon in the Undiscovered egg group or another Ditto.
No Known effect will come. The editor most likly assumed that it would be way to hard to catch a boy and girl of each pokemon you wish to breed.So they made a pokemon who could breed with any pokemon. so no there is no Known negitive effect