Job, rabies immunoglobulin isn't the only thing injected into belly muscles. When my husband was laid up for a couple of years with a hospital staph infection, they were worried about blood clots because for a lot of that time he was comatose or semi-comatose and not moving, so they injected a daily dose of anticoagulant into his belly muscles. When he was awake enough to notice, it obviously hurt. I've seen other drugs administered that way too.
Blood clots in immobile patients usually form in (the deep veins of) the
leg, so I count those anticoagulant (e.g.,
Enoxaparin/
Lovenox) shots as having an apparently valid
reason to go there (in this case, near the large blood vessels leading to the legs). Same for local anesthesia shots combatting pain in the abdomen, or pain that goes
through nerves in the abdomen (a situation occuring more often for gynecologists, I'm told), of course.
Injections that are
popular to go into the stomach region are, in particular, those that the patient administers himself - no need to fold yourself up to reach a leg, twist around for sides or back, or do it single-handed on the other arm. (Insulin shots, which you do into the subcutaneous fat, are particularly often done that way.) That doesn't imply that it's
necessary to place them there, though.
Lastly, there seems to be a hoax circulating among bodybuilders that steroids develop the muscles around the injection site more than those farther away, so those who want an instant sixpack are interested in making abdominal injections. The replies I see posted to
them go out of their way to explain why one would
not want to do abdominal injections unless necessary; in particular, the layer of muscle over your intestines is rather thin, contains more blood vessels you could accidentally hit as well, and if the muscle chooses to
swell up, chances are that that'll be a problem in and of itself because some of them are "caged in" by tendons that
cannot give way (which is exactly why a sixpack looks like a sixpack and
not like just another beer belly).