I have a drobo and told it to tell my mac it has 16TB even tho it only really has 4x1TB drives in it. This is because I keep my itunes videos on it. As I get more videos and run out of space, i don't have the problem of some videos itunes uses being on one volume, and some on another, separate volume - they're all in the same place.
if i had not told the drobo to tell my mac it has 16tb free space, if i added more hard drive space to the drobo i would have either have to reformat the whole thing (ie. reformat the drobo and loose all the data on it), or create an additional volume, which would contain the extra space i'd added to the drobo. i don't understand raid so i don't know what you'd have to do with a raid set-up if you wanted to add extra hard drive space, but the impression i get is that with many types of raid you can't just add space to them anyway, you'd have to create a whole new raid array. i would never bother creating my own raid, so for me the advantage to the drobo is against using regular external hard drives, which would still mean my files would be spread over multiple hard drives as i had to buy new ones for more space (and of course that the drobo protects against a single hard drive failure, or more if you're not using up much of the available space on the drobo and you have more than two hard drives).
while it's true having files on different volumes is not a problem in itself normally for accessing content, since itunes shows them all in the same library, long term this would get really messy if i kept having to add new volumes, so i figured eventually i would want to consolidate some of my older, smaller volumes into one new large one, as hard drive capacities constantly increase. this could be a hassle if itunes doesn't automatically update the library when i move the files from one drive to another (i've noticed itunes does sometimes automatically update the references, but sometimes it doesn't, especially if i'm moving files between drives).
the only disadvantage to this is that Mac OS shows I have loads more free space on my drobo than my drobo can actually hold with the current hard drives that are installed in it - but drobo dashboard, and the lights on the front of the drobo, make it easy to see how much space you actually have left. the drobo software warns you formatting the drobo to 16TB will make your computer take 16 minutes to start up as the OS scans a drive it thinks is 16TB, but that's not been the case - so perhaps that only happens with windows, in which case that would be another disadvantage.