Yes, they dont include variables. They can't predict off-years, career years, or take into account injured years. They do however, give you an objective view on a certain player or team. I wouldn't go as far as saying that they're not a guarantee for the future since you can conviceably(sometimes rather easily) project what a player can do or if his production is set for a decline.

Here's an article on BP that closely resembles the above article, a review on the D'backs moves:

http://www.baseballprospectus.com/article.php?articleid=4692

This isn't directly directd to you nihil, so don't take it like I'm talking to you.:)

Some things to take from the article:

• BP suggests that with the weak pitching market out there, Byrnes and Co. are banking on getting a stronger defense to improve Arizona's pitching compared to last year.
  
• The moves SHOULD improve the defense enough to prevent 50 LESS runs than last year while at the same time slashing payroll and having more flexibility with the money.

- They have Eric Byrnes as being an upgrade defensviely by a lot. It's not that he in fact is that great, it's because Cruz and Green "butchered" center so much, that adding Byrnes would be the equivelant of adding a Gold Glover out there. A suprise to me actually.