All the times I've tried, I just can't rely on an underpowered laptop as my main system. I think I just gave up on that idea. For me, they're best as a secondary system.
Well in what ways are they underpowered for you? I can understand if you need a much bigger hard drive or something that relates to what you do daily. It might be interesting to see what others do that relates to why they picked a desktop over a notebook.
Personally I use my computer for web browsing, writing, listening to music, and keeping in constant access to my email. I could definitely be fine with a 1 ghz powerbook with plenty of ram and have everything I need. My itunes library is 20 gb's and most of my movies are on an external hard drive. I think that currently, the average user can get the same use out of a laptop than they would of a desktop.
If I were constantly editing high quality video files or did very frequent recordings I could see myself leaning towards a desktop. I use my computer for a lot of the day and love the conveniance of being able to do my work in bed, in the library, or at the office. Mileage definitely varies per person though.