[Every time I've ridden it to or from the end, it seemed like I've been the only passenger south of 110th and possible 116th. If it was to make turning easier -- via the triangle -- well, the triangle isn't used anymore and the buses make simple u-turns now.]
Actually, there were two reasons for the extension to Broadway & 106th:
1. The NIMBYs on Claremont Avenue. They objected to buses (then every 30 minutes), but didn't mind constant trucks.
2. The American Youth Hostel at Amsterdam & 104th. The backpackers who stay there have been an important factor in its recent ridership growth.
BTW, I used to live on 106th, near the terminal, and often used the M60 to reach Queens.
And yes, the M60 is supposed to loop around Strauss Park ("the triangle"). The official routing is via Broadway, into West End Avenue, left 106th, to last stop and layover on 106th at the near side of Broadway. (The M116 was even moved to the far side to accommodate the M60.) Any driver who drops off on Broadway at 106th is off-route.
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[And when did it start making limited stops on Broadway?]
When it started running to 106th. It's supposed to stop only at 120th/119th, 116th, 110th, and 106th. (In fact, it makes local stops ONLY along 125th Street and in the airport.) Any M60 signs at intermediate stops on Broadway were installed in error.