I'm A- and have had the injections. I did have a m/c before this one so I guess that's why they are monitoring it a bit more. I've had the injection at 28 weeks like Fee and I know I'll be due for another before this pregnancy is over and maybe another after birth depending on the baby's blood type. I think even if you haven't had any previous pregnancy, m/c, etc, they'll give you the injection anyway. You'll have blood tests to check for antibodies too. That's what the injection prevents.
I haven't had a miscarriage before or any probs. But like you said they just give it anyway. My OB gives it to all Negative patients at both 28 and 34 weeks. Most people I know who are Negative have this done.