Just tried the Wal-Mart Danskin Now padded 4-ft. barbell this morning. It's easier to change weights since the bar is shorter (I am *never* going to use the full extra foot of bar length on a regular 5-ft. barbell for more weight plates!), but I still find changing weights on a barbell a pain. I haven't yet tried it as a body bar (which is how Danskin is advertising it, oddly), so I can't comment on how the weight distribution feels using it for real body-bar workouts. Nor do I know whether the plates on the end might bonk me in the head if I were to try to use this for a true body-bar workout.
I do think that the primary advantage of barbells is that they are adjustable, but they're not fast to adjust. I'd have to think further about whether my idea of nesting body bars would be faster or not. It seems like it would, since it would only be one operation instead of two (both ends of the barbell) to change weights. Barbells are also *much* cheaper (got my real barbell and dumbbell set for $45 at Dick's Sporting Goods, for a total of 110 lbs. of weight, IIRC).
As for substituting one for the other: I wouldn't use the bar of my "real" barbell as a body bar, since it's 5 feet long (close to my height!) and not padded. Too bad, since it happens to be the perfect weight (15 lbs.). OTOH, body bars typically range in weight from 9 lbs. to 30 lbs. I use my barbell in the range of 20-30 lbs. and might someday go beyond 30 lbs. if I get really strong.
So I don't know how well body bars would sub for barbells.
This is what I'm guessing: decide which actual workouts (or types of workouts) you would like to use barbells or body bars for, then decide what weights you would like to use. I know the answer to that question for me for the heavy-weight side of the issue, but I'm still waiting for others to chime in on the appropriate weights for functional-fitness workouts. Anyone?