Having options is good; saving energy is good; improving battery life on your laptop is very good. Having to log out and back in to switch between the two video cards on the MacBook Pro? Um... not all that good. Kinda annoying, to tell you the truth.
Engadget posted a video last night of the swap process (click one button in System Preferences, log out, log back in -- seems ripe for automation) and while it's not particularly onerous, it does seem very weird considering that there aren't any other Energy Saver changes that require a logout. Also worthy of note: the default setting on the new machines is for "Better Performance," using the integrated card.
Why force a logout? Perhaps there are issues with having to redraw windows when flopping from card to card, or particular apps that go 'kaboom!' when told to move onto the other hardware? I can certainly imagine that 3D games or Core Animation-dependent apps might throw a minor freakout if the world shifted under them suddenly, but it still seems like a lot of people are going to settle on one graphics card based on their usage profile (desk-bound gamer vs. mobile maven) and stick with it.
There's also the possibility that the real advantage of this dual-GPU config won't be realized until the OS catches up with the gear. Adding a 2nd GPU is a fairly radical proposition if all you're gaining is the option of better battery life (although, since the 9400M hardware is part of the system chipset, it comes along 'for free'), but when Mac OS X 10.6 comes along it should include the capability to offload general-purpose computing tasks to the GPU, which could provide exponential performance increases for certain apps. Having an extra GPU sitting around waiting to be harnessed starts to seem like a really good idea when considered in that context.