why? because they make me feel worthless.
you need 2.5 times as many people to beat the same guy as 10 man. by that logic, it feels like the 25 people of that group are 2.5 times weaker than the 10 who just killed the very same guy in 10 man, even if that's not really true.
if I were WoW's lead game designer, I'd do it like this:
- bosses have the same amount of health in all modes. other stats may vary;
- raid finder is 40 man, for an army of not-so-skilled random unknown foot soldiers;
- normal mode is 25 man, for a big group of skilled adventurers;
- heroic mode is 10 man, for a tight group of extremely powerful heroes;
that way the game would feel more logic