err, just to clarify, the scouting combine gets you the height and age of all 48 players, something I think is very useful. Group demonstration is the equivalent of scouting all of your 1 or 2 ball prospects (ie. the bottom 19 players based on rumours) once. And if you have done the scouting combine and have more than 9 18 year olds in your bottom 19, then I think it can be a good idea to use (if you then have enough scouting points to further scout the good players for their potential).
As far as computer controlled teams, they are usually atrocious at drafting since they go into it with their pre-set draft list (which is different than yours), and is very close to being just random.