Reasons for a desktop:
RAW Power... As the article explains the best performance is going to be a desktop but often this is at a time when the price doesn't even justify the move for most users. Intel i7s are nice but 'necessary'. There are also the 8 Socket Mainboards that hold 128GB of RAM that just isn't in the laptop world. If you are needing this much power, you probably can afford it and aren't going to want to take it on they Airplane with you are be siting in the park or on a beach while using this level of computing power.
Gaming(Not so much)... Laptops have effectively caught up for several years, but do cost more. And they have caught up for a long time, from 2002 when you could get a Geforce 440, and in 2005 you could buy a Laptop with a Geforce 7950 that would outperform all but one NVidia card at the time. The same is true today, with SLI laptops running the latest GPU configurations. Even for $800 bucks you can buy a laptop that will run any DX10 title at maximum quality at native resolution. That is a heads up to people that might want a laptop but are gamers. (Go look at MSI or Acer or ASUS with ATI or NVidia Mobile GPUs)
Media Center... Sure you can do a laptop to sit next to or under your TV, but you are wasting money as you don't need the screen, keyboard, etc. Desktops here make sense, and the Mini and applicance-like desktops as more of us move to 'Theater' like environments.
Upgradeable or Build your Own... This is another areas where Desktop configurations shine, although there are few OEM Notebook designs that aren't bad, and with MXM and other technologies that offer a modest level of upgradeability, laptops are no longer just a disposable appliance once its performance generation is left behind.
Servers... Even in non-rack environments like your home, a nice Desktop or Mini even if headless is a pretty good option. However, a laptop could be shoved into a server role, as many laptops offer 2-3 drive RAID options and with exteral SATA and good speed even on USB 2.0 you have a lot of storage options.