Fact is every year 32 teams pick in the first round and of those 32 picks 1 or 2 may possibly turn into elite talent. Five might turn into very good and maybe 5 more are just good/serviceable. That's 20 players that probably won't be any good.
So I think the real fact is the draft is a crap shoot, there are very few guarantee's. Sammy Watkins is as close to a guarantee as it gets and if given the chance to pick a "close to guarantee" teams would choose to do that every time.
EDIT: And the reason why they are so valuable is because it increases the chances of landing a very good talent, it does not guarantee it. Just ask the Browns about the picks they used from the Julio Jones trade.