The SSR data will only be comparable if the aircraft is in level flight, the climb report to departures is +-300ft and really can't be any tighter as there are delays in the system. An arriving IFR aircraft from the flight levels may never fly level long enough for a SSR to verify it has the correct QNH set prior to an approach. So with regard to large aircraft operations it is a critical check to ensure correct QNH is passed on. However even though a light VFR may not see the issue, if ATS get picky with which situation they provide it inevitably they will forget to pass it to an IFR aircraft that then proceeds to bust a minima and blame ATS for not updating them on QNH. Rules like this are usually reflective of arse covering as well as safety benefits.