You have to understand that testing the map also comes with rating the map
What's the point of testing the map if you can't express your opinion with a simple vote? And most players are only playing qualified map to get an early lead on the score boards, anyway.
Any map that doesn't deserve its bad rating will have it adjusted to its mean when it goes into the ranked section. Though maps that I've seen in the qualified section that have low rating are generally bad, it won't change anything if you're just delaying when the players can rate a map.
Somebody said that they didn't download maps that had bad ratings.
What is to stop people that played BEFORE that person even saw that map on the list, and it was rated low? The exact same thing, they wouldn't download the map.
Now I'm not saying disabling user ratings is a bad idea, but from a neutral stand point, think about the ones that do play qualified map and rate them legitimately. Do those people deserve to rate the map? They don't really have a reason to play the map again unless for high scoring purposes, and they wouldn't even bother to play it again JUST to rate the map.
A truly good map will never have bad ratings, because they will be outweighed by the good ones. There aren't as many "troll" ratings as you think, because a good map to you doesn't necessarily mean it's a good map to everyone - or maybe even a terrible map to others.
Also side note: I'm pretty sure qualified section is just a reassurance so that the people that get their scores wiped have nobody to blame other than themselves.