Why export to DNG?
Multiple reasons:
#1: my urge to upgrade cameras and to get the latest/greatest is unfortunately more pronounced than my clients'. Often when they want raws their system isn't able to read my files. no problem with DNGs.
#2: I can bake in my looks into the DNGs without sacrificing quality. O encountered many times that either the XMPs are lost for many reasons or that they get scrambled by FTP uploading.
#3: Proprietary file formats suck. Why can some companys work with a common standard (like Leica), but most refuse?
#4: by handing out DNGs of my work, I can still proof my original ownership by retaining the raws. not that I ever have needed this, but I enjoy a bit of paranoia from time to time.
#5: recent developments like Adobe moving to subscription models or the way Nikon handles their NEFs I am even more dubious, even paranoid about proprietary file formats. I wanna make sure that I can access my files in a few years, at least the selects.
I usally keep most of the raw files, but only convert the selects to DNG.