Except that their favorite secondary stats are identical to Protection Tank favored secondary stats... the only difference is slightly reduced Versatility weight. Bonus Armor gives Gladiator stance as much raw Attack Power per point as Strength while still giving tank benefits as well. The talents don't have to change either, as (apart from Shockwave versus Dragon Roar) no DPS-increasing and mitigation-increasing talents share the same tier; you still generally chose simply AoE or single-target.
I'm not saying that WoW's Gladiator spec stands as a proof of concept contrary to what you were saying, only that this example does not work.
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I really don't see why it should be so difficult to balance though, or why one would think that the game forbids a job from being good at two roles at once. Look at the Bard changes for Heavensward, for example; the 'support' slot being basically guaranteed anyways, they seem now to be decreasing the dps gap between support dps and main dps. Sure, the main dps will probably have to have enough burst / alternate support / whatnot to warrant not going double-support in most cases (for the unlimited AoE spam, for instance), but beyond that they're not really competing for slots. More common in recent content, even, is forgetting the Bard altogether due to its lower longterm dps when there are too many breaks in which to recover TP/MP anyways, etc.
That being said, Samurai will not be a 'support' dps so much as a dps that, like any 'main' dps has some utility, which would likely come in the form of being able to snap-tank. He would be the one who could stack with tanks more safely than anyone else (at least with preparation - probably no more safely than a non-BFBing DRG otherwise), he can hold adds, etc., doing anything from reducing necessary stack counts to easing tank grabs (and thereby burst dps checks), to even reducing the number of tanks needed for slightly outgeared content (providing a tank insofar as is necessary without sacrificing basic damage). In many ways that's not so different from any other 'main' dps now, but would certainly see use.