Same at my institution. Generally ~20% of the final grade. The unfortunate reality of the situation is that evals are generally great on most people, so the shelf is really what can make or break honors Vs. high pass / pass or A Vs. B (however your school does it).
This can be frustraing.....what makes one a good test taker of sometimes minute or random knowledge can be at odds with aggressively reading about your patients or staying late to help the service out. It's a fine balance that will take a rotation or two to nail down.
I found studying for the exams a good incentive to add breadth to my knowledge, though, and they are good prep for step II.
Keep 'em or ditch 'em? I would keep 'em, for sure, but try to de-emphasize their overall grade impact. They are, at least, quasi objective and standardized, in contrast with evals. My 2 cents.