Porygon2 is a mid-staged pokémon that's often regarded as more useful in competitive play then its evolution. Even before Evolite was implemented, Porygon2 saw use comparable with Porygon-Z in early DP, due to the fact that it not only had higher defenses then Porygon-Z, but it was also the bulkiest pokémon with Trace available, as Porygon-Z always loses Trace in place of Adaptability upon evolving. Combining Trace with it's respectable defenses and offenses, ability to use Recover plus electric and ice attacks, Porygon2 could take down things such as Salamance, Gyrados, and Heatran with ease, making it highly coveted, until Hitmonlee version released Outrage to Mence, rendering Porygon2 much less effective at countering what it was most needed to. This combined with crazy issues eventually caused it to fall back to the depths of being a situational pokémon only useful for setting up Trick-Room...Until Stunfisk and White came out.
Rising from formally being (quite literally) considered obsolete to one of the best defensive pokémon in the Blanket, Evolite gave Porygon2 the exact thing it needed to be able to handle Salamence again, and not only that either. With Trace and its boosted defenses, Porygon2 can also fight toe-to-toe with Hydreigon, Garchomp, Rayquaza (Try and use Porygon-Z for that, heh) and more! As Trick-Room also gained recognition as a viable strategy with the help of a certain green blob, Porygon2 can also show off its utility as a support pokémon, using its quirky move-pool to the best of its abilities, with an improved Magic Coat and recovery abilities being its main points.
On the converse, Porygon-Z is actually falling bellow Porygon2's usage level from the increased usage of fighting types along with the introduction of Genosect, a pokémon with Download which though hits somewhat lighter, has a much more stable defensive type along with a higher speed. Whether Porygon-Z will see increased usage for whatever reason remains to be seen.
Ironically, one can say that Porygon-Z did in fact make Porygon2 a better pokémon, even if it did so through rather indirect means.