lol.
Say a person has no space at their desk because they live in mom's closet. They absolutely have to have a laptop, and they are a gamer with loads of cash. Then by all means, by the biggest, baddest gaming laptop out there.
I always thought of laptops as secondary systems anyway. I really don't picture them as primary machines. The problems they can have running 24/7, or at least all day. I think of laptops as just additions to desktops. The detachable sidekick if you will. To take with you, use as a laptop is meant to be used. Until you go back to do your work on the good ol' desktop.
And seriously, desktops can come in pretty tiny packages, even with upgrade options. Such as extra half-height PCI and AGP, more plugs, full sized bays etc. Mingled with an LCD and a wireless keyboard with mouse built-in, doesn't take all that more space then a laptop. Plus they are easier to upgrade RAM and CPU or Hard drive later on.
If you want a laptop as your primary PC, to sit at a desk all year except when you take it on vacation. That isn't a good reason to buy a laptop. If you need to travel with your PC on a weekly, maybe even monthly basis, then a laptop may be better. If you can't get by using memory cards.