Well let's see...
If you used one complex carb source, let's say oats, and about 0.5-1.0 lbs. of lettuce daily, what would you be missing out on. How much unhealthier can it really be to eat oats and lettuce, rather than oats, yams, brown rice and lettuce? Show me what nutrients are significantly devoid there.
And even if there are a few (which I doubt, but I'm not positive), a multivitamin and mineral with a high bio-availability can counteract the lack of nutrition. People say the regular food is better, but I really don't see how this is true. As long as the vitamin you have is being absorbed enough to equal the amount of nutrition you would get from the regular food, there would be no difference, aside from the lack of substance in your stomach, but that's not the issue.
The only real issue that would present itself would be the boredom of consuming the same thing day in, day out, in extreme amounts. I could know. Ten servings of oats a day for a month or so. It killed me after awhile. If you can get past that, go right ahead and eat the same complex carb source. Make sure to have veggies. That's where most people miss out on health. Vegetables. Romaine lettuce especially.
Fiber too. Make sure to get fiber from something. Ground up flax seeds provide this along with other benefits.