I don't think kraeusen is detrimental to the beer. At least not IME. I have read that it is good to blow off the headache causing elements in the beer which are formed in the kraeusen, but I'm not sure if that has been proved. Also, at commercial breweries I don't think they blow anything off. I think it the kraeusen just settles back into the beer after active fermentation is complete. As stated, I think the point of a blow off tube is to keep you from having to clean up a sticky mess. It gives the kraeusen extra room to move.