Back to the posters original, original point: I think we can all agree that no matter what the math is, if DM's are either 1) sloppy or 2) perversely sadistic in creating encounters for their particular party, then the game can rapidly become no fun. And for me, as a player, this doesn't just mean avoiding TPK's, it also means avoiding strings of worthless no-challenge encounters.
Yes, CR / ECL and all that business assumes a balanced party, but that can be a pretty darn big assumption at times. Not every party will be composed of players of the same level of experience (I'm talking game experience of the actual players, not the level of the PCs they are playing). Also, not every party will be composed of Lidda, Milalee, Tordek and Jozan, in other words not every party will have the chapter-approved optimal mix of classes. If the DM is running a game for a group of 4 3rd level half-orc bard PCs, then thats the party he has to design his encounters for -- NOT the balanced group that some (not all, but some) of the math "behind the curtains" of the game assumes. A group of ghouls , for example, would tear this particular party up, much more so than their EL would normally suggest (that EL based on the assumption that these ghould would be facing an idealized, mytical balanced party where a cleric or some kind of divine caster is always present and always has a turn undead handy).
GM's who rigidly try to apply EL's / CR's in a cookie-cutter "by the book" fashion without considering the unique strengths and weaknesses of thier particular group of players are really jeopardizing the fun of the game.