I disagree, it's always possible to have a clean UI. I'm a prot warrior with a metric crapton of buttons I use during combat my UI is pretty sparse. I bound recount toggle to a key so I check it during downtime instead of leaving it up. I don't need any sort of scrolling combat text and as I never read it. I have MSBT setup to play sounds on various conditions (such as sword and board) and Ghost:Pulse flashes icons for others.

For example:
In Combat: http://www.ramenite.net/InCombat.jpg
Out of Combat: http://www.ramenite.net/OutCombat.jpg

I have the center set of action bars just to show their cooldowns in combat and they're set to show state "[combat]" in Dominos so they disappear when I'm not in combat. A very simple Hack (name of an addon) script moves the XPerl player/target/targetstarget frames down to the bottom when I'm out of combat.

I use Bartender's paging ability to only show one bar of buttons at a time. It pages to the other bars using shift or alt. Doesn't work well if you're a clicker, I suppose, but for me it works great.

(I recently got ForteXorcist and got rid of SBF debuff timers on the side. I had them doubled up for awhile because I was lazy.)

http://img689.imageshack.us/img689/2553/wowscrnshot020910203720.jpg