I'm about to pay up for a mode S unit but it does seem a bit odd to be paying for something where the primary objective seems to be to implement a system where we can be ignored by ATC.
There's a widely held misconception about the S for Selective bit of Mode S.
It's Selective at the very lowest level of the protocol. With Mode A/C,
every transponder replies to
every interrogation, leading to a lot of congestion. With Mode S, the transponders reply selectively to the interrogation, but the radar still tracks every transponder, effectively all the time, because there are still many replies from each in each second.
Just as with Mode A/C based SSR, it will undoubtedly be possible for ATC to filter out certain code categories on displays, but that is not what the
Selective is about.