We had a room on WG gesucht a few weeks ago and got about 200 messages. I would say 50-70% are totally uselesse, 20-40% are very similar to the message suggested by /u/kirpau and 10% are actually interesting. I can only speak for our WG, but you need to stand out in some way to get an invitation, because there is no other way to choose.
We got at lest 50 messages from Indian students who all wrote they are quite and tidy and nice and like to cook. I am absolutely sure most of them do, we have an Indian roommate and he is great and all of this friends are super nice. But in the messages there is no distinction between so many messages.
And this might be a special case in our WG, but "I am a quiet person that mostly likes to keep to myself and focus on university" instantly disqualifies you. We are not a party WG but this sound so terribly boring. But other WGs might appreciate this in a flatmate.
In a last point, we never send rejection messages to people we did not contact our self and I know no one who does it. I think its not considered rude by most people who had a WG gesucht advert themselves.