The severity levels are there for guidance. I would hope that common sense would prevail, but clearly it doesn't.
If a UI glitch makes a program unusable, then it's normal. I can not believe you need this pointing out to you.
http://bugs.winehq.org/show_bug.cgi?id=1347 "Screen is wiped/blanked on usage of DirectDraw (black screen/desktop)" = UI glitch = Sev. Normal
Common sense people, that's all that is asked.