It's a pain to level barbarians because they just suck at killing monsters in comparison to a hammer or a light sorc. You get bonus experience for making the kills therefore levelling faster but a barb never gets any of them. Soloing chaos is not an option either because it would take too long to berserk everyone therefore you wanna get to diablo as soon as possible to take his experience.
I can see what you mean, But i have never had any trouble leveling my barbs, in baal runs i just use 2*BoTD (any indestructable weapons would be ok) and ww baals minions. Even though your dmg will be lover than a light sorc and hammerdin, there is still a chance that you are the one striking for the final blow and killing.
Even if you just stand in a corner and leech, the difference wount be big.
When i had to lvl my fury wolf, i rushed a druid and sorceress at the same time and did baal rund with them.When they were both lvl 75 i equipped them and started doing my own baal runs with the sorceress while the druid were just leeching in a corner.
So i know for a fact that the difference issent big.