An MD80 primary horizontal trim stabilizer motor failed as the aircraft was being slowed and configured for landing. The Captain continued the approach using alternate trim and landed without notifying ATC; declaring an emergency or completing a checklist.

Narrative

On right base I was hand-flying the aircraft in good visual conditions. Several minutes prior to landing; halfway through configuring the aircraft (I believe at FLAPS 15); I became aware primary stabilizer trim was inoperative. The First Officer did a quick overview of circuit breaker; switch positions; etc. and I asked him to try his main STAB trim switch. Main Trim inoperative; alternate trim worked normally. Configured aircraft for landing slightly early and aircraft trim very manageable and on-speed and in-trim condition for landing. On final I announced my intention to land; and articulated to my First Officer my judgment to land while all set up was safer course of action than executing a go-around and exposing the aircraft to 7-10 minutes of flight with malfunctioning main trim and excursion through wide airspeed regime. Also announced I would purposefully keep trim slightly nose-heavy in the event a go-around were necessary. Landed in good trim and on-speed at Flaps 28. I committed an error of judgment; however. On self-reflection and critique; any primary flight-control malfunction; no matter how well-handled; constitutes an emergency; and in retrospect we should have formally declared same with ATC. By way of explanation-- not an excuse -- by the time we had the situation sized-up executing the plan; we were on short final and I didn't think advising tower had any value. That was a mistake and in the future will declare a precautionary emergency at the beginning of any situation that qualifies. Malfunctions can get very bad very fast.