On one hand, I agree that using legendaries is retarded, but I also hate being that person who dictates how others enjoy their game. If they want to use legendaries, realistically speaking, there is absolutely nothing I can do about it except beat them or lose trying to beat them. In either case, getting mad or acting as though the legendary makes them inferior in terms of skill or whatever doesn't benefit me. The only thing I can really do is git gud and that's all there is to it... unless they ban the use of legendaries in everything except Passerby/Friend battles, which doesn't solve the problem. I'd rather overcome the issue than simply dismiss or restrict the obstacle.
tl;dr people should be able to do whatever they want and if it inhibits my ability to win or have fun, then that's my problem