I don't particularly mind some 'mons being used by several characters, since most of the time this overlap is not with their aces, just stronger tan average 'mons that can fit in pretty much every team even just filling space. What I do dislike is that E17 in particular has gone a bit overboard with the Gen 7 stuff, even replacing much better fitting 'mons (thematically) like Fern's old Snivy.
Also, characters losing their monotypes in this episode (Cain, Victoria, Fern and Sigmund, if I'm not forgetting anyone) is kind of good gameplay-wise, because it makes them more distinct from the regular Gym Leaders and more difficult to battle, but at the same time it's kind of bad for the purposes of the story. As of last episode, all the rivals started the story playing monotype because they were reserve leaders so that's what they were used to, but slowly introduced new types and 'mons as the plot advanced because of things that had happened to them, in other words, them breaking away from their monos was subtle character development, and now that has been erased, because they never played monotype in the first place. Even some in-game dialogues were cut off this episode to accommodate, like Fern mentioning that he lost once to Aya because of the grass vs poison match up or Cain saying that he had specifically prepared a non-pure poison team to use against Radomus. I miss those little things.