Now that Starlight has failed to appear in two episodes that have featured all of the mane six, I do not believe there's any reason whatsoever to proclaim her as the "mane seventh," or even a main character at all. And with the nature of the appearances she's made thus far, I think it's finally clear just what her role is.
I no longer believe that she's been included into the cast so that she could learn about friendship. Her friendship studies and her efforts to become a better, non-psychopathic person are only secondary to her true role. I think that she's been included primarily for the sake of developing Twilight further, now that she's taken on the role of a teacher. I believe that Starlight has only been made a recurring character purely for Twilight's sake rather than anyone else's, including her own.
Little effort has been made to include Starlight among the mane six, something that would actually be important if she were intended to be a main character. I can't help but find it particularly striking that she wasn't present when Rainbow Dash was officially inducted into the Wonderbolts, an event so important that even the CMC were present for it. Each of them knew exactly how much fulfilling that dream matter to Rainbow Dash, and yet as we saw, Starlight apparently could not have been bothered to show her support. Nor, apparently, could she have been bothered to be present to assist opening Rarity's new boutique. These were clearly two ideal episodes that could have displayed her closeness with the mane five, and yet both were needlessly squandered.
Overall, it just feels like Starlight is NOT the mane five's friend. She's only Twilight's friend.
(And Spike's, too, I guess.)
This could be improved if we end up seeing more interaction between her in the mane five, but that would probably mean that she would have to have an episode in which she's independent of Twilight, and I'm really not sure whether or not that will happen.
This could honestly end up going either way, but suddenly, I do not feel that it is beyond possibility that Starlight will no longer be a mainstay after this season (assuming that this season isn't the last, that is). It seems entirely possible that Starlight's absences from mane six ensemble episodes may be the staff's way of keeping our expectations in check by implying that Starlight will not be in this for the long haul. She might end up moving out of Twilight's castle and returning to her town, now with an entirely new outlook on friendship. That wouldn't particularly mean she'll be removed from the show entirely. Her secondary status might be similar to Discord's in which she would show up on occasion.
Or, of course, this could end up going in the opposite direction in which Starlight receives her own Element and/or becomes an alicorn princess in the season finale. We'll just have to see....