My point is that without the phone the N900 would just be one computing device in a continuum of computing devices and, for the price, you could get better computers or, for significantly less, you could get a much slower Linux computer.

As Laughing Man says, the key is that it's a phone plus, a smartphone, a superphone, whatever you want to call it. It's a phone with computing capabilities which exceed those of other phones. It allows you to leave other devices at home, as long as you don't need their full capabilities.

There is no device, yet, which you can use as a phone and then put it in a dock and use as a desktop PC.

P.S. There will be eventually. But by that time we will want something even better.