User blog comment:FairyRaven/Fairy Tail Chapter 427: Pitched Underground Battle, Review and Commentary/@comment-31.132.65.54-20150408015850/@comment-1359322-20150408122100

Just because their goals overlap doesn't necessarily mean their method do.

An example, if Zeref's means to die involve Natsu losing his humanity and become a Demon that could potentially pose a threat to humans, Ultear won't agree and might even oppose Zeref.

And for me personally i wouldn't want Ultear or Azuma to return. Having to write/draw for too much characters also means having to deal with more and more useless/meaningless characters. Just look at Meredy, she hasn't done a thing since she was reintroduced 7 years or 1 year ago. I'd rather see characters doing something significant sacrifice and gets phased off nicely.