I have a Power Mac G5 and a MacBook Pro. I consider the Power Mac to be my 'main' machine, or central hub where I keep all my files etc. The MacBook Pro is used as a day-to-day machine, but in the end, all files created will eventually be transferred over to the Power Mac. It's mainly to do with storage - The Power Mac has much more storage and I can afford to have everything located on it, with only the essentials on the MacBook Pro.
I could probably get by with just a desktop, but the Laptop gives me a lot of freedom