Huh? The point of a laptop is that you can use it anywhere, whether or not you have access to the internet or mains power. You say you want to use it for student things--can't you think of any situations where you might want to use it away from home? Sitting in the library, perhaps, or at the park? You'll never want to go to a friend's place to study and not want to have to lug 10 kilograms of computer with you? You're right, it does sound pointless paying a premium for a portable computer if you're not going to, well, port it... but most people with laptops DO find themselves wanting to move around with them. My four-month-old MBP has never yet left the range of our home wifi network, but even so I use it in all kinds of different places, bed, kitchen table, couch, out on the patio, which I couldn't do with an iMac.
Second, wifi isn't the only way that a Mac can access the internet. Other people in your family and friends circle may not have wifi, but they may have internet access you could plug into. The Apple Airport Express is also very portable and requires no setup to turn someone's wired home network wireless during your visit. And wireless access over cellphone networks in your area may not be laughably expensive like it is here. ;-)
I urge you to think this through and be absolutely certain that you'll never require true portability from your Mac. I don't know anyone who has a laptop and leaves it sitting on their desk all day, every day... but if you ARE going to be one of those people, well, yes, you'll get much more bang for your buck buying an iMac.