Why do you think ntpd provides only seconds or minutes accuracy? This is certainly not true.
Oh, you probably haven't had the problem. But for some the problem is relatively commonplace: NTPd after some time starts refusing to sync time. And no matter what you do (restart HW, restart NTPd, sync manually, and restart again everything) that POS would still within hours again start refusing to sync the time.
And when the NTPd refuses to sync time, the skew easily rises into the minutes. On some buggy virtualizations - even more. (I have said hours - because some VMware versions/configurations I have seen seem to have a bug in time implementation, where guest runs faster(?) by about 1 minute per hour. 2 days uptime == 20-30 minutes of time skew.)
In the same configuration under the same conditions, the OpenNTPd runs just fine.
The f***ed up configuration and documentation of the official NTPd was the main reason why people have actually developed the OpenNTPd. If NTPd was perfect, nobody would have even bothered.