I'm biased, given that I actually made a graphics set.
But compare the following:
Can you really say that ascii makes it easier to distinguish between stuff?
Ascii relies a lot on mere shades of colors to distinguish between different things which otherwise use the same characters. You don't get context clues either. For example, in ascii, all dwarves of certain related professions are blue. There is no way to distinguish between a potter, a leatherworker, and a glassmaker. Are those ganders and guineacocks, or are those goblins? Your fort's survival becomes dependent on your ability to distinguish between exact shades of colors at a glance (say, the difference between light gray and dark gray; or the ability to pick out a dark character against a black background). Guess wrong and the fun factor suddenly ratchets up. The closest you get to a context clue is guessing what Dwarf Fortress thing starts with a particular letter. Is that W for warthog? Or is it a walrus, water buffalo, werewolf, William Shakespeare, warthog woman, water buffalo woman, werecamel, William Shakespeare's woman?
There's also the way these colors and characters interact with each other which makes it look far more cluttered imo. It's almost impossible to pick out something in ascii quickly from a non-uniform background. Sure it's easy to pick out something specific against the large areas of pluses that make up smoothed floors, but how about against a stockpile which takes in everything?
With graphics sets, even if something is unfamiliar to you, you can guess what they are by their appearance and where they are. With ascii, you pretty much have to 'k' strange new characters all the time. If you can recognize them as new characters, that is.
That said, if you do prefer ascii, then more power really. And sure ascii can be beautifully minimalistic in its look, but the reason why people make new tilesets and graphics sets in the first place is because ascii really IS harder to learn and far more ambiguous. And just because ascii is more difficult, doesn't make it superior either. With that reasoning, we might as well say that the honestly atrocious UI we have now is a "feature".