I would like to understand the favoritism towards being able to affix materia to the sentinel gear as opposed to merc/sipaphi/sorc and other rare/ex gear, also being said, why is the sentinel gear not rare/ex?

There have been a number of times I end up helping people farm coffers for gear and yet when we get to sentinel its normally the same but without the rare/ex modification, we found members of our party ended up selling the gear which they claimed to oh so need. I find this very frustrating to exclude gladiators and mrd from being honest when it comes to gear drops and also the exclusive boost to gear modifications because their set is not deemed rare/ex.