That depends entirely on your usage pattern. It sounds like your laptop use fits the "carry on bag" pattern -- you throw a few files in the "carry on bag" laptop, go hop on an airplane, and sync up with the "mothership" later. (That's pretty much exactly the market that netbooks and the MacBook Air target.)
There are other patterns out there. Mine would be more the "Winnebago" usage pattern -- I certainly could keep some of my stuff on a server elsewhere, but I'd rather have everything right there with me and live on my laptop full time. (Sadly, I only have a 500 GB drive at the moment, but that may change depending on what happens with prices.) This also influences choices like the 17 inch screen and full keyboard.
I'm sure you'll find opinions to the contrary, but I'm going to insist that neither way is "wrong". Do what works, but don't expect the same thing to work for everyone.