In the UK TMZ - Transponder mandatory zones - are becoming more common. If you have mode s you can ignore them if you have mode a/c you have to get permission to enter which can be a pain.
I find I get clearance through CAS more easily now I have s over my old a/c - same aircraft.
Mode S is becoming steadily more important in Europe with different states having different rules but it all slowly getting harder to (a) know the differences and (b) fly without s. France is considering changes which may require mode s in class D but this is not a done deal.
In the medium term it is likely that Mode S combined with ADS-B will give considerable advantages for collision avoidance and airspace access. Avoid the Garmin range of units as these will not do Extended Squitter without spending silly money - if at all. Almost all other units will.
Rod1