As of today, I am using my laptop for my carpc also. I am looking into purchasing a used dell docking station for my laptop for the portability aspect alone. Since I purchased my laptop last year, I perfer to use it at work, home, and in my car. So, I want to be able to quickly detach my laptop from my carpc without having to take out all the wires. As I see it, thats pretty much the only reason you would need to buy one.
For me, the remote on switch isn't too important because if you are usually docking your computer into your car when you get in, then its not like its out of the way to turn it on too.
As for power, I have decided to use an inverter and attach it directly to the battery to free up the cigarette lighter. And if you buy the port replicator (make sure it has a VGA port, I guess some don't), then you don't have to worry about lugging around and plugging in the power cord everytime also.