Disagree. I have a family of three. We have a current Mac Pro which is used for all sorts of things including gaming. We also each have a 13" MacBook Pro. We usually buy a single new 13" MacBook Pro each time a new one comes out so that all of our notebooks don't become dated at the same time.
We have two World of Warcraft accounts. Often someone will be playing on the Mac Pro and someone else will log in to the second account from their MacBook. This works just fine today, but I fear next year's model will fare poorer than today's machine for doing the same task.
No one in our house has any desire for a larger notebook. In fact, as they become more powerful, we will all likely move to 11.6" MacBook Airs in the future (we presently have one and love it). We also have no desire for a second desktop, as we only use the one we have for doing things that require a lot of horsepower.
First, It is unreasonable to bring a new product to market that performs less than the machine it is replacing. Second, I know many people who like us don't want to pack around a larger notebook. Third, I know many people who cannot afford a a larger notebook than the 13" MacBook Pro. Many of the people in that third category are regular World of Warcraft players.