Porygon was little more then a harmless little experiment, a simple being of light and data that would result in the spark of something massive. As it has been stated that Porygon somehow contains structures compatible to Mew's DNA within its coding, that must mean that like Mewtwo, Mew's DNA was converted into data used in some way and then manipulated (and locked) to be able to function into a being restricted by its own "genes" (if such a term can even be used to describe Porygon's structure.) From there, experiments rose to higher and higher heights, resulting in the eventual creation of beings like Castform and Mewtwo, as well as various beings which are the result of industrial waste merging with a strange source of light in order to create them, such as Grimer and Voltorb (in fact, Vanillite, Claydol, and many more Pokémon are stated to have originated in a similar manner, possibly hinting some connection).
Pokemon such as Mewtwo, Genosect, and to a lesser extent Porygon-Z are the result of people exploiting such new-found biological and cybernetic engineering, creating creatures which far outpace Pokémon formed by natural means, Pokémon who can hold their own against beings worshiped as deities and feared for their power, and effect the world on a global scale with their abilities. This power however, is limited to a select few individuals, and as each being is either unable to reproduce offspring of equal power or sterile entirely, there are means to restrict such Pokemon from causing damage to their maximum ability.
Another issue other then modified Pokémon goes back to Porygon, and it's apparent ability to reproduce with Ditto. As stated before, Porygon can only function in the ways its program allows it to, making it a clumsy and predictable Pokemon reflecting its experimental nature. But its ability run on energy entirely obtained from external sources for an almost indefinite period, the fact that it has no predators at all, and the fact that it can adapt to just about any environment, all with next to no biological needs, means that any encounter a released Porygon would have with a Ditto could potentially be chaotic to the regional ecosystem. Porygons would quickly over-run entire biospheres, clotting them up and intruding into natural Pokémons habitats, and due to their resilient nature, would be extremely difficult if not impossible to remove in a large enough scale to reverse any damage already dealt. In deed, releases of Porygon in Sinnoh and Unova might some day result in unfortold consequences in the future...
