Exactly. How many of these contracts are already hamstringing teams? Verlander, Pujols, Cano, Lee, CC, Mauer, Choo, Hanley, Wilson, just the vast majority of the following page:
http://www.spotrac.com/mlb/rankings/
1. Players often break down in their early 30's. While this is not definite, it is a legitimate point of concern.
2. These contracts are very large and provide little to no surplus value, can hamstring a team's budget.
3. The risk of a massive loss of surplus value outweighs the risk of a slight surplus value advantage.
4. There are more efficient ways to add wins.
Conclusion: These contracts don't always end poorly, but generally are bad investments.