They are getting characters that have appeared in ONE episode in the series, it's a high school reunion, they want to get as many as is possible before they run out of money.
Not sure I'm seeing the problem here considering they got all the main characters who matter and who could reasonably make an appearance relatively early. Everything else is frosting on the cake.
There is no problem. People just seemed shocked they are getting back characters no one really remembers because they were only in one episode. I saw bring back everyone that fits. All the high school classmates can appear, have a line and not wreck the story.
I just hope we don't bring back Veronica's mom or Duhcan unless it fits the bigger story.