In short: Many reviews show that for many 3D tasks, Arrandale's built-in GPU is between 90% and 110% the speed of 9400M. Yes, there are other tasks where the 9400M blows away Arrandale; but for "everyday use", Arrandale is very close to 9400M. But, there's another thread for that argument; let's not fill up this one with it, too.
My view (regardless of the exact performance difference,) is that having switchable graphics would be the best solution.
If all you're doing is web browsing, word processing, spreadsheet, iTunes, the intergrated graphics would be just fine for those, and you could squeeze out an extra hour or two of battery life.
When you want to do the real 3D work (whether games, hardware-accelerated video encoding, or other GPU-intensive work,) you can switch over to the GPU.
Even if it was back to the truly horrendous GMA 950, I would likely use GMA 950 'on the go' much more often than discrete graphics, just for the battery boost. (Obviously, if you don't regularly use the system on battery more than 6 hours a day, this feature wouldn't matter to you, and you'd just leave discrete graphics on all the time; but I often drain two first-gen MacBook Pro batteries a day, that last 3.5-4.5 hours each. I'm willing to trade GPU performance for battery life. Obviously, I would greatly prefer to have the choice, though.)
