First, it would not logically include level 50 dungeons. Because level 50 is the cap. You can't run a level 50 dungeon on a job that you have over the rank of 50.
Second, yes, I'm sure many people would like to be able to do the level 50 dungeons on easy mode. If people find an easier way beat the level 50 dungeon, I'm sure they'll use it. That has no bearing on whether we should be able to take our r50 jobs into the lower level dungeon.
If we're making a slippery slope argument, be aware that the slope slides both ways. If players find an effective way to get through the r30 dungeon, they shouldn't be able to use it? If people figure out that Thm, Conj, Arc, Glad is the best party set up, should we ban that set up? If the end boss is weak to earth spells, should we make it so that no one can cast earth-based spells? Just because something is harder doesn't make it better.