Mire-
San Juan is right, you do not have to complete an AOA internship if you don't want to. However, you will not be able to practice in 5 states (FL, MI, PA, OK, WVA I believe...correct me if wrong) without the AOA internship. Everywhere else you're good to go with just the allopathic residency.
If you do choose to do an osteopathic internship, in many cases it will not lengthen your total postgraduate training time. Some specialties such as Derm, Anesth, Rad, PMR, etc. require an internship anyway, and actually begin residency at PGY-2. In other specialties such as IM, your internship year may count as the first year of residency at some programs.
So, the bottom line is that doing/not doing an AOA internship depends a lot on where you are going to practice and in what specialty. I have heard very good things about doing one, because it seems like you get good exposure to lots of different areas and you get to do quite a bit. But, to each his own.