I think some folks are confusing 'dangerous' with 'evil' here. Of course magic is dangerous to use. All the more reason to have a system in place where people can be trained to use it properly.
In real life guns are considered dangerous, but try fighting a war without them. In warhammer terms humans need magic to counter the magic of their enemies. Oh the occasional battle wizard may turn to the dark side, but compared wit that the consequences of going to war having no access to magic at all is far worse.