Bert,
Your programmer is correct with your analogy used. An application would be more efficient, and would probably perform better when it is running in a multi-view paneled window such as, MS Outlook. It also depends on how an application is programmed (are programmers using best practices and sound techniques). That's critically important with how the application performs and flows.
I begin most of my designs, by following MS Outlook standards. If a product's design and architecture is consistent, a product has a better chance with good performance (mainly with usability) and good GUI consistency.
The goal to good design is eliminate most multiple windows and mouse clicks. Granted, there will always be some standalone windows to open, like with your Outlook example of sending a new message.
