A very interesting topic, let me add my 2 europence to the discussion, quoting some previous messages and adding a comment.
When I'm flying under IFR, I have no idea much of the time what airspaces I'm passing through. Under IFR, it's largely the same. I don't look, I don't care.
trust you don't do this when you're flying in the USA! Over there it's quite normal to have a mix of VFR and IFR traffic at altitude (typically Class Echo airspace), but in most cases ATC will be able to tell you the reported altitude if the VFR traffic is not in radio contact (they would say "altitude unverified" in that case).
The fact that under IFR you should never have any VFR traffic nearby is very much a European thing, as if all the airspace is always class Alpha. Well, in some cases it really is Alpha, like in the huge TMAs here in Italy...
Mode C transponders have a 100 feet resolution, but if you have a Mode-S transponder, its resolution is 25 feet. Since introduction of Mode-S we often see airliners crossing a whole FIR at FL301 (30050 feet)...
I believe that the 25' resolution is not mandatory for Mode S, and I am sure that a lot of Mode S installations on GA aircraft still use the old blind encoder feeding their gray code altitude via the 12 bits parallel wires, and that only supports 100' resolution.
All the newer encoders support serial data out, and the newer transponder often have an integrated blind encoder supporting 25' resolution.
Now, my comment on using Mode A alone, without any Mode C reporting. Are we really seriously still doing this today? This is madness!
As far as I know, if a Mode A only transponder is interrogated by TCAS, the lack of altitude reply via Mode C will give the worse case scenario to the TCAS interrogator, and TCAS will put the intruding aircraft at the same altitude as the interrogating aircraft. There is no way to escape such an RA other than a lateral deviation, though maybe you are thoudsands of feet separated vertically. You just don't know about that since Mode C is off.
In my opinion, in case of a known Mode C failure before the flight, better avoid flying altogether, and if it happens in the air while in contact with ATC, they will tell you quite soon like some have told us in this thread.
But knowingly using only Mode A, to me is a no-no. Why not just turn off the whole transponder altogether?
Ciao, Luca