ASCII is much more efficient for interpreting tiles. Graphical tilesets are for nonserious roguelike players, which is fine, especially for a nonserious sandboxy game like DF.
Actually, false.
I played with ASCII up until there started to be so many more animals. Now, you have a single letter, such as "s" representing dozens of completely different kinds of creatures, from satyrs to sharks to snakes to skunks, that having a picture is actually much more efficient at simply determining what a creature is.
You can tell not just what general "class" of job a creature has, but you can immediately identify specific jobs, as well, since your beekeepers will actually be in beekeeper outfits, rather than simply being a brown generic farmer-type unit.
Furthermore, you should check out what
Stonesense has been experimenting with lately - they base their graphics off of memory-hacking and have a fully custom way of displaying information, so they are capable of now doing things like displaying what a dwarf is actually wearing with different .pngs, as well as coloring them based upon what dyes the clothing has.
Hypothetically, they will also be able to display what tasks a dwarf is accomplishing or what dwarf status is through the same instant-recognition icons that ASCII is simply incapable of displaying.
ASCII is a barrier to the amount of information that can be conveyed to the player in a single screen's worth of information, as there are only 256 possible uses of any given tile, and there are already fairly limited numbers of tiles on the screen already.