For me personally I don't NEED a DRO, however, as others have said they are very nice to have to do accurate work quickly. The Grizzly DRO's are over priced IMO. Look at the DRO's on Ebay. I've owned a few of these and they are just as good as the name brand ones.
Speaking of the name brands, I have an Accurite DRO on my mill that was purchased around 2000. l recently had an issue with the X axis scale so I called Accurite. They were not particularly helpful other than saying they no longer support my unit but could sell me another scale that is not a drop in for $700. So the myth of buying a "brand name" product with the expectation of being able to obtain future support did not hold up in this case.