One (for me) hassle with the ever changing RAW formats is the need to upgrade software all the time. I used to buy about every second iteration of Adobes software, and every second iteration of Capture1. The same went for Microsoft office (to read files other folks send me).
Because I now upgrade my day to day cameras with every iteration, I ofthen have to buy every iteration of software due to raw incompatibility with previous software generations (ACR - Adobe Camera Raw, being the prime example.) If camera makers wrote to a DNG format, this would presumably be easier, even though there are frequent updates to that format also.
My costs are basically up by an small sum because of proprietary formats.
Other issues are software/firmware tie-ins to operating systems. When Apple upgraded their OS a little while back, Canon's stopped working tethered to Capture1 because of software issues. This was a disaster to me, as my assistant just pressed the "upgrade" button on the laptop I use for location work, and a number of following shoots had to be shot to CF card due to tethering problems (I always shoot tethered for the client to see the work progress and be able to provide feedback to me.)
I wholeheartedly support open file formats and standards, on everything that produces files. For me the only true answer to how much DNG factors in is if one of the big two (canon/nikon) would offer DNG on their very top model. Only then would we see if it had any impact on camera choice.
On a side note about standards, even attatching .jpg files in emails are a hassle. Clients with windows machines ofthen complain of "not being able to save" the .jpg from a mail attatchment. So I have to make a .zip file as the attachment, and then they dont understand how to open the .zip, etc. etc. Life would be sooo easy if there was an accepted open standard for compressing files across operating systems. A lot of clients don't even understand how to download and open .zip files if I make a download link for them. They do such things as click on the temporary files to open them before they are downloaded, try to move the temp files before they are downloaded, etc.