I'm probably going to get a bunch of downvotes for sharing a perfectly honest and valid opinion, but, I just really cannot take seriously someone that develops on nothing but a laptop. I don't even know how you physically manage to deal with lugging such a thing around all day or dealing with the ridiculous keyboard.
I have been developing my private stuff on nothing but a laptop for the last couple of years. I actually switched from desktop PCs to notebooks and I'm very happy with them.
You should put a regular keyboard, mouse and screen at your desk, which eliminates the keyboard and mouse troubles and gives you a regular two monitor setup when at home. On the road, you want at least a small notebook mouse and a notebook with a resonable keyboard. The notebook keyboard should have a numpad, because otherwise the Home, End, PageUp, PageDown and cursor keys are probably inaccesible, and they are absolutely necessary for writing code. With that setup you can be just as efficient as with a desktop PC.
The weight can be a problem for 17 inch notebooks, but already with 15.6 inch I would bet that the printed scripts for the lectures weight more then the notebook. I used to carry a 17 and later 15.6 inch notebook to university every day and it wasn't a big deal.
Also, when on the road, don't forget to pack the two most important tools of every software engineer: A piece of paper and a pen!
As to the question if there is any real benefit to a notebook as opposed to a desktop PC:
If your work requires you to be hooked up to the company network, a bunch of console devkits or an industrial robot etc. then the mobility of a notebook won't do you any good, that is correct. But for a student the situation is different.
As a student you go to lectures and either they are important, or it's a bunch of stuff you already know, or the guys who is giving the lecture doesn't know shit. All three cases can and will happen and in the latter two you want your programming gear around so you can utilize the time by educating yourself. Same goes for empty timeslots between lectures, which is also very common at universities.
When you have visited all your lectures of the day and go home, you probably have some exercises to do, or hobby projects to work on. Maybe you don't want to do them at your own desk at home, but rather in a park, or while visiting your parents. Or maybe the exercise requires you to join forces with a fellow student so you meet at his place or in the library. In either case the mobility of a notebook is a very big plus.