If the May Green tileset didn't replace letters in standard text with the new symbols, I would likely use that over ASCII. I like ASCII, but a clean, easily readable tileset is just as good, if not better (such as the time my dwarves were being harassed by skeletal groundhogs, which were the same color as the dead grass, making them nigh impossible to quickly locate).
A lot of people seem to mistake "Presentation Arc" for "3D Graphics," and of course those people would be wrong. Making the interface more navigable - universal mouse input, standardized menu commands, more intuitive interfaces and multiple possible views (e.g. in-game Dwarf Foreman) - is what the arc is all about.
Adding support for modders to specify custom tilesets would let the community create and maintain its own graphic sets to keep up with Toady, rather than forcing Toady to keep the graphics updated himself. It would probably resolve a lot of the hassle that would otherwise occur if Toady tried to set a graphical standard in-place before the game itself, since there are more of us than there are of Toady and a majority of the players like the graphics just the way they are. For those that prefer tilesets, they can make and update their own.
If Toady opens DF to specific graphical assignments for each and every entity, you can bet our nearly obsessive-compulsive community will create an image for each and every possible tile and combination, from skeletal groundhogs to burning miners to dead ettins. The default should always be what Toady has right at this very moment, pure ASCII; simply adding support for others to build a prettier world would be more than enough for a lot of players, myself included.
(Edited due to massively long sentence)