Probably for the same reason you think it's bad? Bad SMNs will lead to bad impressions. In SBCoB I pretty much parse equal to or past our MNK as SMN. This is my ignorance shining through, but if I remember correctly there are some individuals who parse do not include the SMNs pet when it comes to average DPS throughout a fight. It's either they forget to or they don't think it counts, which, as a reason is stupid.
As for mostly seeing BLMs in endgame raids, that can hardly be used as proof as for why SMN is bad.
