I have relied on my PowerBook G4 for nearly five years as my computer for both work and pleasure. It's powered-up for most of the day, and it's proven to be a good arrangement for me. Last year, about this time, I orded a 20-inch Apple Cinema Display to complete the "desktop" look, and it has helped for reading documents, which is an activity I do a lot in my work, and with its extra screen realestate, it has been great for laying out documents in InDesign.
As you mentioned in your post, the other big advantage of having a notebook computer act as your desktop is that your computer is portable when travel is required––and that is a nice feature that no desktop system can offer.
If you rely on just your notebook, I'd recommend getting a stand, an external keyboard, and mouse. But it sounds like you're wanting to integrate and external display too, which I think is to your advantage.
I hope this is helpful.