I really don't think it's a question of balance. Really all the jobs in ffxiv are balanced extremely well, you can do any fight with anything and not be set back all that much for using a technically non optimal setup. There have been sam's that have cleared ultimate, and the dps check is quite tight. The issue though is that if you take a sam the rest of the group's personal dps numbers will not be as good as they would be otherwise. This means that they post worse numbers to fflogs, and because of that make everyone else look worse than they are. This means sam, blm, etc are just not wanted. Not because they aren't good, not because it holds the group back, but because there are real consequences for the other players in the group. You just don't want a situation where players can think "I'm getting nothing personally for accepting this class".