Would you otherwise give him cow's milk as a healthy drink? Breastmilk is obviously more appropriate and healthier for a human child than cow's milk, so if you are happy with the idea that cow's milk has desirable nutritional qualities, you should be happy that breastmilk confers even more.
For example, kellymom quotes a study by Dewey (2001) which suggests (I have not read the Dewey study) that "in the second year (12-23 months), 448 ml of breastmilk provides:
29% of energy requirements
43% of protein requirements
36% of calcium requirements
75% of vitamin A requirements
76% of folate requirements
94% of vitamin B12 requirements
60% of vitamin C requirements"
These nutritional qualities presumably remain present in breastmilk after 2, though the proportion of each nutrient, etc., will diminish in relation to the increased volume of solid food a 2+ yo will eat.
kellymom also cites a number of articles which discuss the immunological qualities of breastmilk, even beyond 2:
"Nursing toddlers between the ages of 16 and 30 months have been found to have fewer illnesses and illnesses of shorter duration than their non-nursing peers (Gulick 1986).
"Antibodies are abundant in human milk throughout lactation" (Nutrition During Lactation 1991; p. 134). In fact, some of the immune factors in breastmilk increase in concentration during the second year and also during the weaning process. (Goldman 1983, Goldman & Goldblum 1983, Institute of Medicine 1991)."
For my DD (2.11) at least, the benefits of bf-ing seem largely to be psychological or emotional. Nowt wrong with that as a benefit, I suppose!