How dangerous is to take-off and fly with snow on the wings?
Pilots are allowed to commence a takeoff if the snow is not adhering to the wing and will blow off during the takeoff roll.

It is very difficult to just look at a snow covered wing and know for sure if the snow will blow off. A "hands on" tactile inspection should be conducted and even then you can never be sure about the level of contamination for the entire wing surface.   De-Icing is always the safest option.  

To attempt a takeoff with snow and/or ice adhering to the wing is NOT SAFE.  You are gambling with your life, and the lives of your passengers and crew.   

That being said, it is possible to do a reasonably safe takeoff with snow and/or ice adhering to the wing, but you will need a much longer takeoff roll, and slow rotation, and climb out.  If you were to have an engine failure you would probably not have sufficient performance to climb out over obstacles, or even remain airborne. 

You are gambling that you have enough runway for your "seat of the pants" takeoff and that you will not have an engine failure, because you have no way of knowing the actual aircraft performance.    

So again, De-Icing is the only safe option.