Also, what does this "more cluttered" refer to? To my knowledge each tile and the objects on it are displayed exactly the same way whether you use graphics or not, only the symbols are different.
I find Mayday's set to be too cluttered - the ground, the boulders etc - like there's too much detail in each square to allow the eye to take in the entire screen. The default tileset and other tilesets which just change fonts slightly keep the incredibly clean and easy-to-read look over nearly any size/resolution of a screen. I look at a Mayday screenshot and have to ask myself if there's anything special about that ground, or is it just normal rock/earth/walls.
A tile is displayed as a tile, true, but the two tiles (default and graphical) may be so very different that the game fades into illegibility at times (one example which irked me about a tileset was that each "." would become a spattering of earth which filled the entire tile, and appear at the end of every sentence. And that the "=" sign would be a pile of logs, even when it was being used to denote the quality of an item).
That being said... I prefer square. I'm really bad when it comes to symmetry and making sure everything is measured. The only things that have broken me of this somewhat are the OCD challenge I ran myself and playing a co-op game using DFterm.
But that's only because you are gotten so used to ASCII. Give the two options to a tabula rasa person and they'll definitely find most graphics sets easier to interpret.
Have you tried to read sentences filled with switches, spatterings of dirt, buckets and grass clumps? It's oddly distracting. (and no, I'm not exaggerating. It's the one reason I eventually dumped graphics sets)
DF is the only ASCII game I've ever played. I came straight from 2D and 3D gaming (eg WoW, Aion, Spore) with a strong sympathy for sprite-based games (especially RTS and RPG). If anything, I'm the person that graphics sets are aimed at. And I found that the more tiles I could see at once, the more confusing the entire layout was to me. When we could only see few tiles to a screen, it didn't matter that tiles were cluttered because they would be viewed at a low resolution (SNES games, for example, on an old TV). As screen resolutions become higher, more and more tiles can be viewed on a single screen, and with the variety of things that are being shown on one screen at once in DF, it can become harder and harder to take in a scene at a glance. Mostly the clutter that I find so distracting is in backgrounds - the ground, grass and water graphics of sets.
I get that it is an aesthetical choice for everyone who chooses to use or not use a set, but to claim that I only defend the original ASCII because I am "so used" to it is fraudulent. I have only used the vanilla tileset since .31 came out, and it was then that I realised how much easier it made reading the game - for me.
Edit: Again, I'm not saying that either is superior, or the 'only' way to play the game, just musing on what I, as one player of DF, has come across. I believe whichever way someone prefers to play the game is up to them, and can only suggest that experimentation is a great way to find out which type of tileset/graphics set suits one. For me personally, that was starting with graphics and a custom tileset, and ending up using vanilla. As Tormy says, once the doubles issue is sorted out, a much wider vista will be opened regarding graphics sets.