While laptops are very affordable, you still get more bang for your buck with desktops. Only the most expensive laptops can run games like a good desktop can. For most things, my $1000 laptop keeps up with my husband's $3000 mac desktop pretty well, but he can run four instances of world of warcraft on his computer. Mine wouldn't be able to handle that. My laptop IS a desktop replacement though. It's heavy, has a big screen and poor battery life. I don't care, since I always keep it plugged in. I just need to go from my apartment to my parents' house.
You can hook up a large monitor to both, but really, if you're going to be using your laptop as a desktop computer most of the time, you're better off with a desktop.
Desktops last longer. It's easier to replace parts, monitor, keyboard, you don't have to worry about the battery dying or dropping it.
So really, it depends on how you use your computer. For a lot of people, desktops make more sense than laptops.