I grew up in a rural area AR-15s are very popular for varmint hunting. Predators like coyotes are a real problem for farmers. Some states go as far as to offer bounties for the purpose of population control.

http://en.wikipedia.org/wiki/Varmint_hunting

To your original question, do you "need" a semi-automatic rifle for varmint hunting? No, but they are very well suited for that purpose.