Those of you who complain he's back, well maybe you should pay more attention to the story line instead of being a bunch of whiners that he returned.
Pay more attention to the storyline? If anything, I'd say the people who are displeased by his returns are the ones paying the most attention. Shadow Mario is an awesome character, certainly. But even the coolest characters can overstay their welcome. Now, if you look at the series as a whole, how much sense do his resurrections make?
He spends a good deal of season 3/the movie trying to get his hands on the Truth Stone. Mr. L even calls him out on this, saying he's not immortal without it. Then Wario kills him... And he resurrects himself, not at all surprised that this has happened. While it's possible Mr. L just didn't know what he was talking about, why would the Darkness put so much effort into obtaining a stone that makes him invincible if he knows he already is?
Then Waluigi comes and gives Wario a spell. This spell removes the Darkness' invincibility and make him just as killable as anyone else. It works, and Wario kills the Darkness again. In OBS, he even admits he's dead, and that he doesn't care what happens to him because he is. At the time, this made sense, but looking at his recent actions it no longer does. If he had a way to revive himself again, why would he risk destroying himself? It's possible that will be explained/retconned in episode 60, but there's still the issue of why a spell that was specifically designed to destroy the Darkness failed to do so.
During season 3 and the earlier part of Act II, the Darkness was my favorite villain. But every time he's brought back the reasoning behind it gets more convoluted, and I think that in the long run it just hurts the character. He also just hijacked the plot from Verestorm, which means that people who found him to be a more interesting villain (or even just a nice change) are inevitably going to be upset by it.
While I will wait for episode 60 to call this a bad decision, I have my doubts that Richie and co will be able to explain it adequately enough for me to call it a good one. But we'll see. Maybe they'll surprise me.