they could have still done character specific episodes, and have the friendship lesson told to Twilight like they did in "Stare Master" where Fluttershy learned something instead of Twilight.
also, I feel the lessons are not always friendship-related during some of the episodes. last weeks doesn't count, as it obviously had no real lesson /applejack
I think they just didn't want to make it seem like Twilight was the only one learning about friendship
but the show is really about her, Twilight Sparkle, learning about the magic of friendship because beforehand she was an over-studying anti-social pony.