People just want to have fun while they play. Sometimes people want to make a challenge for themselves with a risk of losing - it makes the rewards more worthwhile. Like a Nuzlocke Pokemon challenge, speed runs are a way to challenge yourself and your party's skills. I agree that it should be decided upon beforehand, though, but in a threatless world, sometimes you gotta try and make some excitement for yourself. But you can't say it's a cancer on the game when it's considered other peoples' way of feeling challenged, rewarded, and excitement, and ultimately having fun with their playstyle.