I find it really hard to get the full suite of vitamins and minerals from my diet. I exercise a lot, but have a relatively sedentary job. My eatting habits are generally quite good, but I still find it quite difficult to make the RDA minimums on a 1600 to 1800 calories. And even more difficult when I am restricting calories to drop a few pounds.
I tend to think of a well rounded multivitamin as more of an insurance plan to make sure my body is getting all it needs to be in peak performance.