This has been my observation. I own a 13" MBP with the 1280 by 800 screen. It is plenty adequate for a single workspace and gets the job done.
When I'm in school though. It's mediocre. When doing engineering spreadsheets, my professors are extremely particular. During class, I use my MBP to work on with the screen full of Excel, half of the 24" PC monitor for their directions, and half of the monitor for a PDF with the equations I'm typing into my spreadsheet.
When I'm home, I'm using a 32" 1080P TV as an external monitor to do the same thing. The extra screen real estate makes me much more efficient. Rather than jumping back and forth between windows like the other students, I'm able to work almost 50% faster.
Now I'm looking for either a HR 15" MBP or 17" MBP. The size may not be as convenient, but the efficiency while working would more than make up for it for me. I want a 15" MBP at 1920x1200, or a 13" MBP at 1680x1050. Hopefully Apple will offer us those options soon. They may not be a popular choice for most, but if Apple can do resolution independence with the next OS it would make those resolutions a workable possibility.