The problem isn't so much with spinoffs, when the canon main characters aren't really on the radar much. To give a Star Trek example -- L.A. Graf's books tend to follow the Uhura + Sulu + Chekhov group, rather than Kirk + Spock + McCoy. This is perfectly fine, and in fact I adore those! It's lots of great character development, and some original characters, and it's a really fascinating fresh perspective.

No, the ones that I'd be annoyed with would be like having the bridge populated by cardboard stand-ups of the whole crew, having all the action on the bridge, and having a guest star who manages to take over the Science station, the Communications station, and the Helm, as well as Trollop of the Week (the original role she was cast in).