Hybrid classes - SAM & RD MAGE.

[Ashkendor]

Tank DPS is usually balanced by making their DPS mid-tier and having their Defensive Cooldowns/oh shoot! buttons being mid-tier as well. Pre-RoK Berserker on EQ2 fit this, doing about the same DPS as a Swashbuckler back in the day, since it was an AoE heavy class like Swash, and their oh shoot! buttons had to be pretty well-timed.

The way FFXIV is set up, that just makes you a mediocre DPS and a mediocre tank.

That means that generally they wouldn't be the preferred choice for either role.

Tank/DPS Hybrid could work if it is made to where it is only filling one role at a time, too. Forced Stance like they're making Astrologician do maybe? If you switch stances you cannot switch to the other mid-combat?.

This has possibilities, though.

[sirDarts]

The way FFXIV is set up, that just makes you a mediocre DPS and a mediocre tank.

That means that generally they wouldn't be the preferred choice for either role.

Well a Tank/DPS hybrid (does more damage than a tank, but tanks less than a tank) could work in 8-man or raid instances where one (or several) tanks are mostly off-tanks, who don't stand in so much of a damage, and their DPS is important. Dunno.
But yeah, in 4-man instances they would be mediocre at best.