My MBP has enough power to run OS X + two VMs (Debian and WinXP) so everything I could ever need to do, I can do on my MBP. (For gaming, I installed Win XP on a second partition and boot to that when I need to get some FPS in.) And since I bring it to work and use it there too, it's basically the only computer I use from day to day.
(Of course, the downside to this is that it's also a single point of failure. Just yesterday I got to work and discovered my laptop's screen was blank. It turned out to be the infamous NVidia defect that created much commotion last year. But, even though I was 6 months past the warranty, Apple is fixing it free of charge: http://tinyurl.com/ctz3h9. Luckily, I also create bootable backups on a fairly regular basis.)
On the big screen issue, I've found it has less to do with the physical size of the screen as it does the screen resolution and your multi-tasking habits. If you really need more screen real estate, use the Unix screen utility or some form of desktop switching with keyboard shortcuts.