In the UK we don't use it as an identification tool, we use it for the parameters that are downloaded from the aircraft.
I don't know if there are any plans to allow Mode S as an identification method; certainly haven't heard anything about it.
(You may already know this, but if you don't read on)
Primarily Mode S (in the UK) it is used to display the pilots selected level and we see that on the radar at all times. Hence we know which aircraft have mode S and those that don't just by looking at the screen, a mode S equipped a/c would have a label that looks something like this:
ABC123
136 LL 150
The flight level after the intention code (coloured orange) is the selected level. This enables us to catch any errors and stop potential Level Busts. Furthermore we have tools that utilise the selected level to help us in congested airspace eg. Holding stacks.
There are some other parameters we get which help when controlling, those being IAS, GS, HDG & Vertical Speed.