If you are going private I think you HAVE to have an Ob.

If you go public it's unnecessary in a normal pregnancy.
If you have a complicated pregnancy and would prefer continuity of care then I'd go for an Ob.

I went public, no problems during pregnancy, and when I needed an Ob at the endof my labour the hospital just called the Ob on duty, who came in and also performed the emegrgency c/s, and the hospital paed and midwives did the once over on Jazz after the c/s. I think thats all a private Ob would do, as they aren't a labour support person IYKWIM. Oh and the postnatal care, which was done by my midwives. A hospital Ob never saw me or looked at my c/s wound after I left the operating room, the midwives were great with that and an Ob would have seemed like a bit of overkill.