Any melee build will have multiclass dips, because there's no reason not to and because its efficient.
For example:
Rogue 1/Ranger 2
Barb 1/Fighter 2
Swashbuckler 3/Rogue X
Paladin 3/Fighter 2
One of the bigger problems is that FIGHTER is a 2 or 4 level class, maximum. The new feats in PHB2 make it more useful for longer, but not that much. Also, melee types don't have any spellcasting progression to keep up. So, any fighter melee type will be looking to mix in something else, if for no other reason than to boost their saves. Barbarian is great if you meet the alignment prereq.
Also, the big weakness of most barbarian-dip builds is (A) MAD (you need high dex, as you're in light armor... but you also need high str and con... and your will save sucks, so you need high Wis as well), (B) Feats (Sure, extra rage helps, but its another precious feat slot), and (C) Terrible Will Saves (short of multiclassing into Occult Slayer or burning yet another feat on Iron Will, its tough to get up with this sort of multiclassing). Also, the upper level barbarian abilities such as Tireless Rage are actually fairly good -- although given enough extra books, you can do better with PrCs (Frenzied Berserker anyone?).
Spellcasters should almost never dip barbarian unless they're going for a special PrC like Rage Mage.