Have you read your stuff and thought you could easily make it an OF? Because if you only use one of the guys why not just make it a random pop star? What is it about the guys or guy that has you keep them as the main character? Is it because you feel very comfortable writing as one of the BSB? I'm just curious because I honestly tried to write a story where it was just Nick as a main character and having the guys barely in it and there was actually a female as a lead and I got about a chapter in and thought, this isn't a BSB fanfic. I'm making this a random pop star and changing it to an OF.
Only a time or two. An old sci-fi of mine, and now, with Undead, I think it'd be okay as an OF. But I use all five of the guys in both (well in the case of Undead...it's We do lol) as well as their personalities. I don't write a story where it's JUST Nick, or JUST Brian. There's always at least one of the others as a main somewhere, if not more.
Sometimes it's a case of I want the Boys, I see them in that character role. And if it's an AU, it's usually because it's not realistic to have them be a famous music group. (It would be weird to have the Boys as BSB and have them happen to survive an apocalypse...o r happen to be secret agents, etc.) I do think it's odd to have one Boy and virtually no sight of the other 4. They should be in there somewhere, even if one is the main focus.