Oh it's no biggie:

1. Not EVERYONE has all classes leveled.
2. Not EVERYONE is interested in Rogue/Ninja
=Not EVERYONE will be leveling them

2. Currently Melee DD queues actually aren't that long...they are typically on par, or a wee bit slower than healer queues. The reason, there are more RANGED classes in the game than Melee. All adding this class/job does is make Melee queues more inline with Ranged Queues (which will probably speed up a bit since as you say, there will be a fair amount of folk leveling Rogue/Ninja)

-The DF doesn't actually just do
-Tank
-Healer
-2 random DD's

It actually tries to get
-Tank
-Healer
-Melee DD
-Ranged DD
Now, yes if queues are Ranged heavy (which they are a lot atm) sure you'll see Bardx2 or if they are Melee Heavy Drg/Mnk in same group. But this is actually not common.

In 8 man content, the game even tries to give you 'no doubles' Meaning it tries to make sure you only have one of each specific job...although again, this is dependant on balanced queues.

-Currently, the DPS queues are NOT balanced (2 Melee vs. 3 Ranged)
-So by adding this class, it'll make the wait times for ranged (which have been long) better.

3. There is currently plenty of reasons why tanks/healers and ranged folk would like to level-sync dungeons....and this desire isn't going away anytime soon.
-Atmus Books
-DR Myth bonuses
-Leveling
-Glamour Gears
-Hell even spiritbonding

4. They allow you to queue for a dungeon however you please if you pre-make the group. At least for low-level dungeons I see little reason why you couldn't do them 4x Rogue w/ cross-class skills that help w/ healing or for safety 3x Rogue w/ Healer friend.

5. There is a chance (albeit small) that level cap will rise around this patch....this will cause LOTS of people to want to level LOTS of jobs....even if just for 5-10 levels.

6. Leves and FATES still exist and still give viable exp....and you can do them while you wait on a queue