I first started learning about DF by reading Boatmurdered.
Using the descriptions in the writing, I was able to puzzle out the screen shots and understand what I was seeing, but it was hard, and in my mind, ugly.
When I started playing I went right to tilesets using the LNP. I found there to be a world of different, everything looks like what it should, and the few cases where they dont or an image is reused I only have to learn a small number of 'conversions' instead of a huge number if I used ASCII. I have also used a few different ones and while a few things change (mostly what various rock/gem/ore tiles look like) its pretty easy to know what doing on with any of them.
Plus if I show off my fortress to a friend, I dont have to explain what every single symbol is, it is so much more intuitive for first time viewers. I would hate to try and show an ASCII fort to a complete newbie as opposed to a tileset.
What I really wish would happen to solve this once and for all, is for Toady to select a tileset to make official as the default (and not have ASCII as the default), or to have one made that uses elements of the most popular (with some back end work that fixes the current limitations) however I doubt he would want to even start on that before essentially everything else is done.
I see no reason to promote ASCII use, I personally dont like the general standard of all screen shots be in ASCII, though the concept of a standard is a good one.