For us folk who don't fly Kingairs or TBM850s and don't hold in the London TMA stacks, perchance what is the 'extra information that is worth its weight in gold' ?
Currently downlinked via Mode S from suitably equipped aircraft are selected flight level, indicated airspeed, heading, groundspeed, rate of climb, rate of descent and flight id. Coming soon is set barometric pressure setting.
Mode S also allows certain other developments on the radar display itself that, for technical reasons, would not be possible without Mode S being used.
These allow a selection of current safety tools to work very effectively with new ones also in the pipe line. These significantly enhance the safety of the overall ATC system. Whilst they may not all be totally relevant to you in your light aircraft I assume you do want to see an overall safe(er) operation. I imagine most Cherokee etc pilots also take the odd commercial flight.
To the best of my knowledge only Scottish and London Centres are currently using Mode S data, the latter does include all the approach control units for the major London airports.
Chevvron may not like it, but the inescapable fact remains that Mode S has since 2005 realised some of the most significant safety benefits to the UK ATC system for many a year and is here to stay.