My first computer was a desktop, and I really loved it. I loved that I could come home from school and use my computer. I loved that it had power. Yeah, it sucked that I couldn't take it with me, but I didn't really notice. I ended up finishing high school at boarding school, so a laptop was needed. Despite having a laptop, I still used my desktop everytime I went home, which was every weekend. Why? It was a better machine.
Over time I used my laptop more and the desktop less. It wasn't because I preferred the form factor, but more because it just made more sense... if I had the opportunity to use the desktop more than two days every week, it would have been a different issue all together.
When I left school and went to uni, I took my laptop. I didn't even consider taking my desktop because it was, by this time, old. My laptop struggled on a very regular basis and was a POS. By this time I hadn't been using a desktop for a long time. When I replaced my laptop (with my first mac), there really was no point in considering a desktop. I was a student - everyone else had laptops.
After a while I realised that wasn't quite true. Yeah, heaps of people had laptops, but many (most) of my classmates were still living at home and had a desktop that they had used through highschool, and still used as their main computer. Their laptops were just for working on campus.
I also worked out that working from a laptop wasn't the best way to work. I soon bought a monitor and an external mouse, keyboard and, later on, trackpad. When I worked out that this setup worked well, I soon stopped taking my laptop with me everywhere. Yeah, it sounds lazy, but unplugging all the devices and monitor wasn't really worth the effort. I had numerous external hard-drives in addition to everything else. If I wanted my desk to look nice and have decent cord management, I'd need to buy another charger so I could leave my charger in place. It just didn't work as nice as I'd like.
At this point in time, I decided that I would be buying a desktop when I moved off campus. It wasn't necessarily going to be an iMac (in fact, a major reason I wanted ta desktop was for upgradability). A desktop would mean I wouldn't have to unplug my printer and harddrives on a daily basis and wouldn't have to worry about whether everything was plugged in properly. I wouldn't have to untangle cords everyday.
After a year of primarily using my laptop on the desk, I started to take my laptop to the library. Of course, after a year of running straight off power, my battery was pretty much rooted. Yeah, I could have replaced it, but I didn't have the money.
That was a year and a half ago. I've moved off campus but I haven't bought a desktop... yet. My old laptop battery life got so dodgy that I stopped even trying to take it with me. It wasn't worth the effort. I had acquired an ipad and tried using that for writing any essays or classwork I needed while on campus, and it didn't work. Sure, it worked great for some things, but not for this purpose.
My laptop died a month or so ago and I strongly considered getting the new retina mbp. I didn't. I decided now was my chance to get a desktop. Of course, I do a lot of work where I need a laptop. It's not a matter of wanting to use one, or preferring one, but needing. I looked at a few of my friends who have a laptop and a desktop and many have those little netbooks/small laptops instead of a full-featured one. I decided to get an 11"MBA. It is small and can fit in my handbag. It fits perfectly on lecture tables (anything larger doesn't fit well) and it means I can take a computer with me to the library. Yeah, it doesn't have a heap of features, but that is why I chose an Air. I've always said I wouldn't want an air as my main computer. After using one for a month I still stand by that sentiment.
I'm getting an iMac. I am goign to try and wait for the new model, but if there isn't one available a few weeks into the semester I will get the 2011 model. I want a desktop. My laptop will be reserved for working away from the house, or on the rare occasion that I want to do something on my computer outside of my room. I will be syncing everything with dropbox/icloud/usbs/whatever works best for me (haven't fully decided this yet). I figure I'll get a longer life out of a laptop used this way. After having a huge iTunes library take up a giant chunk of my old laptop's hard drive, my new laptop won't have the same issue. I'll finally be able to use my mac the way I want to without worrying about runnign out of hard drive space anytime soon. Furthermore, it's goign to encourage better study habits; with a laptop it is too easy to go and work in front of the tv or on the bed. With an 11" screen vs a 27", there will be no way that I will choose the tv over the desk.
Oh and I did some sums and worked out that I can come out better financially by buying an iMac and an air... even if I get a 27" (which I'm not 100% sure I will do yet)... you can't beat that!
tl;dr... I've been laptop only for such a long time and I don't want to be constrained. I want the best of both worlds. As a student, an iPad doesn't work for me (in terms of laptop replacement... it certainly has other uses). With the current technology, why can't you use both?