I like to play games when they are challenging. so instead of using powerful creatures, such as tyranitar, and not caring about the iv's, ev's, and using sheer power to survive [ubers and semi-ubers]; i use pokemon such as houndoom, ariados, a starter, fearow, and ampharos to win. with pokemon such as those, i have to use ev training and get good iv's and movesets, even to survive just the gymleaders, elite four, and random NPCs. my sixth guy on my team is another way i suvive: depending on what i am about to fight, i select a pokemon from the wild that has a type advantage over some of my upcoming enemies, ev train it, and battle. occasionally, using this method, i do indeed lose [most often to the elite four]. but that does not make it any less fun for someone like me. all it does, is make the game more of a challenge, and with challenge comes fun.
so on my stand to ev/iv training: without it, i would lose a whole lot more, would spend even longer to beat the elite four [using things like mirror move and thunder wave], would be laughed at as my sister uses similar statagem as the one stated above [similar because one of her guys is a tyranitar and the rest are things like gyrados] and would have less fun.
those ivs really do make a huge difference as well [some of you are apperently bashing them].
"how do you know?" you may ask:
occasionally, i'm on my last guy, and i win, because i survived by ~5 hit points [sometimes less, sometimes more].
if i did not have max individual values in hit points on most of my guys: then those extra 5 hp would not be there for me to survive.