Diminishing Marginal Productivity

While drinking down at the cowboy bar in San Juan, I confessed my doubts about some of the materials in Microeconomics! My confessor, El Doctor,absolved me of my sin by pointing out that some of my doubts have a basis in fact.

The idea he attacked most eloquently was the idea of Diminishing Marginal Productivity. He argued that some ideas in principal courses are obselete.
Is Diminishing Marginal Productivity an old obselete idea?
I was a little intoxicated but his argument in a nutshell was Microeconomics as taught in Principal courses is just mental gymnastics, but he also argued that there is a value in having students practicing a bit of logic.
Some of the ideas that we agreed were accurately portrayed in Micro were:

• Cost Benefit analysis
• Incentives
• Game Theory