I have very recently leveled up both a WAR to 40 and a PLD to 35ish.

I have to say the dungeons were so amazingly easy to hold threat on that a tank stance would have been overkill. The RARE occasion when threat was an issue was when a DPS was synced down from max level. I would tell them there was no way I could hold off them, usually followed by some sort of acknowledgement that it did not matter, and away we went.

Giving MRD and GLD the tank stance from their advanced job could also create issues in the future if we ever get any other DPS focused jobs that stem off from MRD or GLD as their primary class. You would have a DPS that could go into tank stance, making all other tanks obsolete for 90% of the content, pushing out significantly more DPS while still holding threat and having a mitigation boost that would be enough to run most group content.