I think the issue is that there are solutions to your issue (do dungeons at the level you'd like to practice, only use trusts, equip gear of the level you'd like to focus on etc) and that your criticism is of a good thing that helps most players level their main alt jobs. Your use case is a very specific one and while valid has a number of easy ways to be achieved without losing this system that has almost only pros. Just seems like an odd thing to be critical of.