"or do I know that by instinct and knowledge allready.. or dare I say commen sense...."
No, not everything can be learnt by common sense. I signed with my DD, and at 13 months, she was able to tell me that she was scared of the dark. I had never realised, it had never even come into my head, until she started signing "light" one journey home in the dark. It had been a period of night waking, and I had tried everything to get my DD to go to sleep, b ut that night, it clicked and I left the light on and from that night onwards, she never had another disturbed night's sleep.
There are so many other stories, that I could repeat, like I never knew how much my 10month old loved washing machines until she used to sign to ask to sit in front of the washing machine! In her hey day, she was using over 100 signs - everything from washing machine, to asking to go to see the ducks, from telling me she wanted toast / yoghurt ... for her lunch. Stuff, that you just simply cannot guess alone.
Further, it did not inhibit her language either. By the age of 2, she had over 500 words in her vocabularly (I know, she was doing a university research project at the time, and I had to record her words), and most of those words she first signed and developed from there.
All the research also shows that baby signing helps to develop a child's language skills too. Before you knock it, try giving it a try - after all, as a professional surely you want to do what is in your mindees best interests?
(PS. It takes minimal effort too. I never attended any classes, I just consistently did the signs as I spoke - so it became second nature to us).