“It seems to me that, as used by ATC today, Mode-S offers absolutely no advantage over Mode-C to us GA pilots - am I missing something?”
There are only 2 radars in the UK providing “Mode S” ident (although the feeds go to more than two ATC units). There are no plans to update most of the radars according to local military and civilian controllers in my area. Right now there is no VFR pilot benefit in the UK of Mode C over Mode S, and the only issue is the international situation which appears to be mostly ok depending on which arrears you look at. With Mode s units dropping in price, and possibly starting to be replaced in the US, giving us second hand units, the cost of fit is likely to continue to drop.
Life for non mode C may start to get difficult depending on the TMZ consultations, but that is another issue.
Rod1