Robbins is probably overkill, and if you actually knew the full editions of Nelson's and Harrison's, you'd be teaching the attendings new things. So, no, more in-depth knowledge is not necessary. At least, I never got pimp questions anywhere near that intensity, and the shelf for medicine certainly didn't require detailed knowledge of a huge textbook (I never even looked in Harrison's - I just used Step Up, Pretest and Case Files, and I did fairly well).
If you want to impress, read up on the latest guidelines for the treatments your patients are receiving. It sounds a lot better to say "Recommended antibiotic treatment for this infection is this drug for that duration" than "How about a week of Cipro?" I've had at least several attendings ask things like "What's the literature say about this?"