

These reasons may not matter to most people we all work differently. Forcing all input images into DNG prevents me from making that mistake again and also helps me approach the editing of jpegs with a different set of eyes. I'd find myself accidentally editing my jpgs in Ps before I had exported them to psd's like I had to do with the raw files. This only became a concern for me recently after I had to start dealing with multiple raw formats as well as jpgs.

(I crash Ps on a frequent enough basis that sometimes I'll forget to resets the preferences, so the less I have to worry about them, the better off I am.) For instance, I don't have to worry about whether or not my 'open jpegs/tiffs with ACR' options are set correctly in Lr, Ps, Bridge, and ACR because it doesn't matter all new images are DNGs. If I convert everything (jpgs, tiffs, different raw formats) to DNG first, then everything is much cleaner from an automation and workflow point of view for me.
