However, I was wondering why noone uses laptop computers to do robotics instead of a microcontroller? It seems like Pentium-4 laptops are a dime a dozen nowadays.
well, for starters, the first thing that pops up in mind is 'size' and 'weight'. since many competitions have a cap on the maximum size /weight your robot can be, it hinders a compact design.
greater weight also means that you need more powerful actuators and batteries to meet your specifications
secondly, the cost factor....if a p4 laptop is a dozen a dime, then simple micro controllers cost hundred a dime, plus cost saving on actuators and batteries..
thirdly, the computation power of a p4 is more like an overkill for simpler robots.
Last, assuming it can be done, will the experience I get from the arduino (C programming) be useful if I make a transition to computer-based robotics? Or should I go straight to a laptop based robot?
yes, it can and is being done. and yes the experience you will get from arduino will come in handy. if you dont have any previous experience in robotics , then i would recommend that you kick off with arduino or any other micro controller..because its basically much more easier, the architecture is much simpler ( you would have to learn the computer architecture in order work efficiently with laptops)
if you start with a complex thing straight away , there is a chance that you might not get it , get fed up and give up( not necessary, just saying)..Best of luck in what ever you choose...!
P.S: laptops are usually used as a second stage computing station, when robots are controlled by controllers and they send data to laptops for further processing or commands. if you know some basics , then perhaps you should start a project like that.