Of course Togashi "could" have, but I don't see how the stories we got were any less valid.
He plans things out pretty darn well already. Kurapika and Leorio are fine characters, but I don't understand your fixation towards them.
I said "should" have. And yes, the stories we got were fine for the most part (aside from some other issues I have with the series that I won't get into), but in the grand scheme of things it's still disappointing to not see more of these characters that we've become invested in. It's quite similar to the lack of screentime and development given to the Konoha 11 in Naruto. And while Togashi does plan out his stories pretty well, for the most part, character balance, I think, has always been a weakness of his.
If you call someone a main character because of their screentime and focus, then turn around and say "why don't these guys get more screentime and focus: they're main characters!" then arbitrary is an apt term.
I'm not sure you understand what "arbitrary" means. I believe you're trying to imply that I'm using circular logic, which, no, I'm not. Their roles and focus in the Hunter Exam and Yorknew arcs and their appearances in promotional art indicates that they are major or "main" characters. It's their lack of screentime after which that doesn't make sense.
Bias: The inclination to present or hold a partial perspective at the expense of (possibly equally valid) alternatives.
Your bias is the conclusion that Kurapika and Leorio's presence is generally beneficial to Togashi's stories. I don't think that's necessarily true, largely because your opinion seems built upon the concept of "main characters." What about Knuckle and Shoot, or Genthuru or Knov or Hanzo? They were all fine characters: should Togashi go out of his way to add them to his latest arc? You say that Kurapika and Leorio could have been in the Ant Arc, but that arc was chock full of characters as-is.
You're totally ignoring the context of these characters and their roles in the story. Knuckle was a great character and I'd love to see him in future story arcs, but I'd understand if Togashi didn't. There's nothing really in the story to suggest that he's a main character. Shoot and Knov were horrendously underveloped and mostly forgotten (again, going back to that character balance thing) so I can't imagine too many people are dying to see them again. Hanzo, again, fine character, wouldn't mind seeing him again, but it's not essential. And Genthru was the friggin' villain of his respective arc! Of course he's probably not going to show up again.
This is Writing 101 and I've given various reasons why Kurapika and Leorio can easily be identified as main characters. The point is, when a writer establishes their main characters, they should find a way to insert them in the majority of the story in a way that makes sense and is interesting. And if you introduce these characters and then do nothing with them, then the audience becomes confused and disappointed.
And if there's any bias I have, it's the bias of good writing.
If a story is good, I find it hard to be dissapointed in it.
A story can be good, but still disappointment you at times. One Piece is my favorite manga and Oda has still disappointed me with some of his decisions.