This is a very interesting topic.
Today I can safely say that Barter is only a relevant skill in any way if money is an actual un-exploitable and limited resource in the game.
That still wouldn't make the act of bartering itself fun, but it would justify it's existence and make the choice to invest in Barter a reasonable and rewarding one. Also, Barter is different from Speech in that it gives the PC knowledge of trade and the economy.
But there's a noteworthy problem with Barter and that's specialization and progression. While many players would disagree, I would actually advocate that skills encompass several different fields of knowledge and/or ability. I'm happy that a Melee Weapon can be a Super Sledge or a Knive, two totally different pieces.
There's of course the possibility to regulate this by for example merging skills and perks to the point of common level of efficiency and effect on the game world. But that discussion's worth it's own topic. I'm sure there's still going to be a certain distinction in Bethesda's new system.
I don't want a knive skill, I don't want a seperate First Aid and Doctor skill, I don't want diversification. I want to make the choice of and investment into a skill to have immediate, noticeable consequence. I want clear specialization as in limited character-progession related resources, not in lack of reasonable choice. And yes, I want skills to overlap sometimes. You know, like all combat skills naturally do. Still, all skills need more uniqueness than sameness, except for one: Barter.
Barter can almost never yield unique consequences by definition. Barter has the inherent weakness in balance that it must allow to trade for goods that you could otherwise create (by using another skill) or find in the worldspace and thus save the money for the stuff you can only buy. With a balanced monetary system (no caps overload or easy exploits) the stage is set: There can only be so much room for unique rewards. Barter touches on a lot of other skills and has a broad range of effect. That's it's role.
Of course you could Barter your way through the final boss or Barter for unique items that nobody would ever sell to anyone (who wouldn't steal it/take it from their dead body, rendering the uniqueness almost moot) but you can get only so far without making it feel contrived.
You could also enhance Barter by making a lot - or even all - of the actual trade content in the game only accessible/in any way successfully approachable through investing in Barter. Maybe this would be a good option, but I sincerely doubt it.
I'm still convinced that Barter, although uninteresting on it's own, is a nice skill that should remain seperate (in whatever shape or form) from Speech. Both at least have a connection through Charisma, which is reasonable.