Interesting that US regard 243.00 as 'Mil Common'.
In the UK it is UHF Guard, and monitored by a number of organisations including the 2 D&D Cells. They 'own' the frequency from a ground perspective, and I would not transmit on it (under normal circumstances) without calling them first. The most usual reason for us transmitting on 243.00 is to correct a mis-dialled frequency change. Since all mil ac monitor UHF Guard this is extremely useful to sort out a lost comms situation.
What I don't understand is why Civvy aircraft are not required to monitor VHF Guard. In the current climate, I would think it at least a wise precaution, especially given certain 'airlines' seem rather lax in checking-in when changing from sector to sector during transits.
If you are trying to check-in with the next sector, what is the procedure if they don't respond? Go back to previous? VHF Guard? Some people just seem to plough-on regardless!
I thought it was mandatory to be in receipt of a radar service in Class A/B airspace? How can you if you haven't established 2-way with ATC for the sector?