This is one of the few things that separates this game from the pack. Not many games can you change roles midway through an instance.

Personally, I think it makes things less monotonous. I enjoy being on one job for part of a run, then switching later on... makes things much more interesting.

What's the point of having all the classes/jobs on one character if you don't have the freedom to switch between them. Particularly when that's been a core mechanic in FFXIV from the start.

Just because some Monk somewhere only has Monk leveled and some idiot group wont invite him because they can't comprehend doing something at slightly less than peak efficiency (which, if you're in a pick-up group is nearly impossible to do anyway) doesn't mean we need to restrict every other reasonable player in the community from being able to freely switch between jobs.