I was going to say that completely defeated the purpose of the optional quest, but using it to obtain otherwise unobtainable pokemon isn't a bad idea.
I remember back in Diamond, my first pokemon game and the only game I completed the national dex in, I asked for friends for help, including temporarily loaning me legendaries... which was practically all of them, except for 7.
Even though it was really hard without any previous games and took over a year to slowly gather unobtainable pokemon from different friends, I felt as though this allowed me to enjoy the connectivity features in the games to their fullest.
Despite the GTS being filled with bad trades, with patience, it can be used to find pokemon you can't find. For instance, I remember post-story mode, trainers used stuff like Arcanine and Sandslash (which were added to my seen, but not caught list.) Although some trades were impossible, by using filters (only show a certain gender or level range), the GTS showed me different trades. I checked daily, and slowly accumulated around 40 pokemon through the GTS which I otherwise had no access to.
Anyone who has access to Pokecheck should have access to online communities with people who are willing to help. But I understand that when it comes to "loaning" something as rare as events-onlies like Meloetta and Arceus, that it would be hard to find people who trust you. I'll admit I got pretty lucky having helpful friends. If a person is having a hard time getting the last few pokemon, using Pokecheck is understandable.