My PB Ti is used almost exclusively as a desktop replacement. Pretty much it does not leave my desk, but the times I have thought it would be good to work on it sat in the park or set up in a library, I just thought what a pose.
So why get a laptop? If you moved between two desks i could *kinda* understand but considering the premium you pay for laptops - why not just get a desktop if you're not going to use the portability?