A BE35 pilot reported he felt it was unsafe for MRY airport to use the same ILS frequency for Runways 28L and 10R.

Narrative

I was the safety pilot (SIC) on an instrument currency flight practicing multiple approaches to MRY. I believe that it is somewhat of a hazard that the localizer to Runway 28L and the ILS to Runway 10R use the same frequency. The PIC requested the localizer for 28L and we verified that the Morse code identifier was correct for the 28L localizer. At some point as we were being vectored the Tower switched to ILS 10R. The 'NAV' flag on the HSI popped up briefly; but then went back to normal. It would have been easy to miss this. Of course; the Controller didn't clear us for the practice approach until after Tower had switched back to the 28L localizer. However; the fact that we'd listened to the Morse code had been negated by the switch to 10R. This would be a problem if we had lost communication and proceeded to the IAF after identifying the localizer; but before they switched it back. I flew with this same PIC about a year ago where we were on an IFR clearance to the same airport and flew the ILS 10R even though they were landing 28L. We needed the clearance because of a fog bank to the west. We descended into the clouds on the 10R ILS. We came out of the fog bank a few hundred feet above DH. Right as we came out; the HSI flagged that it lost the localizer signal and the pilot immediately executed the missed approach. He informed the Tower that we lost the localizer signal and the Tower apologized for switching it back to 28L too early. It seems dangerous to allow the Tower to essentially turn off the localizer while someone is on an approach. Worse; if the pilot somehow didn't notice this; the localizer would eventually come back on (so the flag would go away) but now you'd be on the back course without knowing it. Because it's the back course for the opposite runway from what you originally set the HSI to; it would be reverse sensing even with an HSI (which is normally not the case). It seems like a safety hazard to have the same frequency used for two different approaches to the same airport.