If Sear was not there, it wouldn't be an issue of what class to invite. Putting Sear in is basically saying "LNC ONRY". It is asking for people to not get near Ifrit. True you can say "Well get better healers.", but the problem is, people always want the easier way, and having Lancers only is that easy way.

When they design a boss fight, they need to make it flexible for everyone, they need it so people say "Okay get anyone who can DPS!".