Why would a NetWorker customer use a VTL over an advanced file type device? Not everyone agrees that you should.
If you're interested in this topic, I would read this discussion on the NetWorker Mailing list. A lot of good points are brought up by a lot of smart people. I will summarize my position.
Unless you're only using the disk device as a staging device that only holds last night's backups, buying a disk target at this point without de-dupe is a waste of money. And if you're only doing disk staging, perhaps you could spend a little more money and hold a whole lot more using a de-dupe device.
So assuming you're going down the de-dupe target route, should you use a VTL or Advanced File Type device in NetWorker? I think the post will show that it comes down to choice. There's no longer any licensing advantages to one over the other, and (as long as you're not only doing disk staging) copying data from disk to tape isn't any easier or harder with an advanced file type device vs a VTL. (Except for some VTLs that can automate cloning from disk to tape for you, but some in the post were against this idea.)
Therefore, I think it comes down to choice. Bring in the various vendors and ask them what advantage(s) their method has.