I hope you can take a little bit of what everyone has said, because most have valid points. To answer your question about why the patient was on the drip would take a little more history that what you supplied, but obviously the infection, TPN, and the NPO status would all lead to a fluctuation in the patients blood sugar level. Like many people stated above tight control around or slightly above the normal limits decreases infection risks, which is a huge concern in the hospital setting (especially long-term). As to why not use oral meds...Is the pancreas functioning well enough to make any insulin at all? If not there goes the oral meds. And like others have said SC would not provide adequate control because it doesn't act as quickly as the IV and the FSBS checks would probably only be done Q4H.
I guess I didn't really add anything, but maybe I summed it up a bit.