I am not intending to be a judge of comments, but maybe my absence from high end audio for 25 years gives some useful insight. Maybe not
The need for a good DAC in the old days is inarguable. My old Sony D-555 (last higher end item purchased) demonstrated that to me really well. There is a ligitimate debate about their usefulness for more mainstreamers (kind of advanced amateurs like myself). My 52 year old ears could not detect any difference between a commonly sold and fairly respected USB DAC unit, and a typical MB-based HD audio chip. My wife (her hearing also pretty good) and I sampled a variety of CDs for over an hour using two identical Grado SR80s.
Regarding headphones, the purchase of the Grados - upgraded from a cheap ($75) pair of Klipsch ear plug phones and midline infinity speakers - provided a drastic improvement in quality over both.
So I interpret all as this. For budget limited or conscious consumers, MB based HD audio (by now on nearly all desktop boards) will eliminate the need for a USB DAC. Assuming a decent analog signal is available, for anyone who cannot afford high end speakers and is for some reason relying on crappy headphones, upgrading to good headphones will provide an enormous and worthwhile difference.
Oh well, maybe writing that was just a therapeutic thing for me...