Simply because the whole OS is not DPI independent, I don't want to be squinting at small text, menu bars, buttons, ect.
The 17in MBP just has too high of a DPI IMO for what I want to do. I suppose I need more face time to adjust but I came from a 1440x900 17in iMac.
One of these days, OS X will be resolution independent just like Windows is, and then the higher resolution the better! I can't wait for that day!