We carry TCAS to help identify other aircraft, whether on the same frequency or not but transponding. Despite costing more than some gliders (!) it is by no means infallible. Aircraft not transponding don't appear on TCAS.
Who is "we" in this context?
I would doubt the majority of IFR capable GA aircraft have TCAS. Of the people I know who regularly fly IFR/IMC in G airspace only one has TCAS in his current aircraft
AFAIK...