John Sickels had the following as his top 10 hitting prospects before the 2006 season began:
1) Delmon Young, OF, Tampa Bay Rays
2) Jeremy Hermida, OF, Florida Marlins
3) Ryan Zimmerman, 3B, Washington Nationals
4) Brandon Wood, SS, Los Angeles Angels
5) Andy Marte, 3B, Boston Red Sox
6) Carlos Quentin, OF, Arizona Diamondbacks
7) Alex Gordon, 3B, Kansas City Royals
Stephen Drew, SS, Arizona Diamondbacks
9) Prince Fielder, 1B, Milwaukee Brewers
10) Howie Kendrick, 2B, Los Angeles Angels
You can see the full top 50 with writeups here: http://www.minorleagueball.com/2011/1/26/1953803/top-50-hitting-prospects-from-2006-in-review
So, there are some hits and some misses, even with the best of the best prospects. Of those 10, only 2 lived up to expectations - and it definitely wasn't the top 2.
I'm not posting this because I'm down on prospects - I'm not and I'd rather have more and higher quality prospects.
But teams that suck for years and years, ahem the Royals ahem, in an effort to collect prospects (by trading legit ML talent and by sucking enough to get high draft picks) are not doing themselves any favors.
It's great that the Royals have lots of prospects. I'd like the Nats to have that many. But the Royals were just abysmal for 2 decades. They just traded their version of Strasburg for what looks to me like peanuts. And all those prospects don't guarantee them squat.