We could spend quite a bit of time debating this to no purpose so I'll concede right away that if we are talking about single engine GA aircraft under $250k, none are equipped with TCAS/ACAS. My apologies. This misses the point.

The idea of not turning your transponder on because few of us have aircraft equipped to see that transponder is misguided at best. There are plenty of fast aircraft flying in non-ADS-B rule airspace that are equipped to see you so it's not just a "shall" as pointed out above, it's also a good idea.

In addition, as already stated while ADS-B towers do not interrogate mode A/C transponders, they do see the replies when these transponders are replying to interrogations by atc and ACAS/TCAS equipped aircraft. Those ADS-B towers then broadcast that information to all ADS-B IN equipped aircraft. Again it doesn't matter if no one around here has spent the extra $1k on ADS-B IN, those aircraft are out there and having your transponder on will at times make you far more visible to them.