Several studies have shown that multivitamins can actually boost the IQ of children.  Furthermore, there is a study  showing that many of the vitamins are associated with improved memory and mental scores. I will let the abstract in this study speak for itself: "Several significant associations (P < 0.05) were observed between cognition and concurrent vitamin status, including better abstraction performance with higher biochemical status and dietary intake of thiamine, riboflavin, niacin, and folate and better visuospatial performance with higher plasma ascorbate. Concurrent dietary protein in 1986 correlated significantly with memory scores, and serum albumin or transferrin with memory, visuospatial, or abstraction scores. Higher past intake of vitamins E, A, B-6, and B-12 was related to better performance on visuospatial recall and/or abstraction tests. Use of self-selected vitamin supplements was associated with better performance on a difficult visuospatial test and an abstraction test".
The reasons for this are many, but suffice it to say that the brain is a huge nutrient and energy hog. It requires, among other things, lots of the B vitamins. Plus, as people age, they often lose GI acidity and the ability to digest certain vitamins, including the B vitamins, and minerals. A good multivitamin, therefore, looks like a good insurance policy for you grey matter.
There is another key area where a multivitamin may help: immunity. One study has found that a multivitamin with around a 100% of the RDA in minerals as well as vitamins can protect against colds and flus. Several studies have shown that in seniors, whose vitamin and mineral digestion has often declined, a multivitamin/mineral supplementation can help prevent infections.  This was backed up by a recent study that showed that even those in the 50-65 age range can get an immune boost as well. 
NOTE: Many cheap multivitamins have fillers such as Calcium that actually slows or halts the absorption of much of what is in the multivitamin! In addition, some of the standard multivitamins are highly compressed, which is why you could load it as a round if they put a little powder in it, and thus do not dissolve fully.
CAUTION #1: There is some caution in order when it comes to multivitamins. For exammple, folic acid plays a pivotal role in the synthesis and repair of DNA, which is why it's recommended for pregnant women. However, it appears it can stimulate certain cancers in both men and women and there is also some evidence that the folic acid added to vitamin supplements is much more potent than the folate found in food. The bottom line: be careful not to consume fortified breads and cereals in addition to a multivitamin.
CAUTION #2: There is also evidence that too much selenium can raise cholesterol.  Men need to be careful here, because cholesterol is a risk factor for heart disease and endothelial dysufunction, which will translate to poor erections eventually. The study above showed that those with the highest selenium had 8% higher cholesterol and over half of those in this category were taking multivitamins. 
1) Arch Intern Med. 2009;169(3):294-304, "Multivitamin Use and Risk of Cancer and Cardiovascular Disease in the Women's Health Initiative Cohorts"
2) Personality and Individual Differences, 1991, 12(4):351-362; Personality and Individual Differences, 1991, 12(4):363-365
3) Am J Clin Nutr, 1997 Jan;65(1):20-9
5) Chandra RK. Influence of malnutrient supplement on immune responses and infection-related illness in 50-65 year old individuals. Nutr Res 2002;22:5-11
7) Am J Clin Nutr, 2010 Nov, 92(5):1251-6, "Multivitamin use and the risk of myocardial infarction: a population-based cohort of Swedish women"
8) American Journal of Clinical Nutrition, Aug 2008, 88(2):416-423, "Serum selenium and serum lipids in US adults"