I think what Awful is trying to say is. Say you pick Thamaturge / Black Mage to start the game with. You get to lvl 15 and you're introduced to the Duty Finder and your first dungeon. Now it's been a while since I've started the game but I don't believe you're ever "introduced" to the Party Finder. That person then uses the Duty Finder every time they get a new dungeon all the way up to lvl 50. They probably even spammed dungeons to level as well. I did... Anyway they get to 50 and now there is a quest that opens up allowing them to play the Blue Mage. They do the quest, get to level 15, go to try to use the Duty Finder to grind dungeons to level but can't do it. Now they have to ask around or go on the internet, provided the game didn't tell them when they unlocked the job, to find out that they can't queue into the Duty Finder. They either grind regular mobs and probably fates. Or they figure out how to use Party Finder and either sit in it waiting for a generous tank / healer / other dps (probably another Blu) to help with the dungeon, which would probably take hours considering there is nothing in it for tanks and healers to do low level dungeons since you only get tomes and such from the Duty Finder, or shout in the hub areas for a party like people do in Eureka.

Now lets say they stick with it and get to lvl 50. Assuming that this is their 2nd job they have leveled just after getting their first job to level 50, they are not told they can't go into Heavensward and level to 60. They can only do anything level 50. which is Binding Coils and some Extreme Primals that most people only do unsynced for WT. So even if you do the primals you are still going to be a lvl 50 with a bunch of level 70s killing the thing probably faster than you need to see the move you are trying to learn.

In conclusion Blu is not being designed as a job that's intended to be played the way everything else is be playing. It's a job that's there for players who have already been through the content on another job and just need something to do in their downtime. Which is bad design when people want to take Blu "through" the content when the content is relevant. Not A patch or SEVERAL patches later.