I now have a "TCAS" system installed and it's interesting what shows up, and what doesn't.
There are several times more Mode C/S contacts (for which I get a relative altitude) than Mode A contacts (no relative altitude shown).
I know people have all kinds of reasons for not installing Mode S (even though it compromises the ability to fly into Europe) but used Mode C transponders are going cheap.
Apart from the paperwork issue (an FAA 8130-3 is not good enough for a used part going into a G-reg, though there is reportedly a way around that using an EASA Part F company) it would seem sensible to put in a Mode C because radar units everywhere can see your altitude and can provide you with a meaningful traffic service. Plus the increasing numbers of planes which have TCAS can avoid you.
I love mine. It works very well and also gives you something to do enroute when you hear ATC refusing a descent to a Jet due to traffic below, you can see them overtake you and then come down. I have 'seen' lots of traffic well before getting an advisory. I think the safety advantages overcome any freedom or privacy concerns. Cost, for mode C, can also be modest as you say.
The "privacy" concerns are bunk anyway. In Europe, you are always tracked on radar, unless flying really low. Obviously getting an official radar service is very different from being tracked, but if they want to find you badly enough after you did something naughty they will eventually collect the tapes and find you.
Funnily enough, old ATC tapes get sent here for storage, according to a post a few years ago after which somebody from there reminded the poster (who was an ATCO) of his Official Secrets Act signature 
One way, one is reliably informed by colleagues in the discreet cargo business in South America of avoiding being tracked is to buy one of these
I would bet a pound to a penny that nearly all of the aircraft flying only with mode A actually have a transponder which is mode C capable, and are only missing an altitude encoder.
In some cases it may be impossible to fit an encoder, but it's about the cheapest bit of avionics kit both to buy and fit; some people are just stingy.
What additional information does your TCAS unit receive from mode S transponders compared to mode C?
What additional information does your TCAS unit receive from mode S transponders compared to mode C?
Nothing 
In that case I don't really understand the privacy concerns. Obviously mode S discloses your registration to ground stations, which might make some people feel uneasy, but if TCAS units can't receive that then I don't see the cause for concern.
If someone doesn't want anyone to know who they are and what they're up to, don't squawk. If you want to avail yourself of some kind of air traffic service, you'll have to tell the unit who you are and what you're doing - in which case the most capable transponder is surely the best thing to have? I guess cost is always going to be a factor though.
Mode S discloses your reg to ground stations if the transponder is correctly programmed, which not all are.
Mode S also discloses your reg to some TCAS systems, via (I believe) a decode of the 24-bit ID which in the case of N-regs maps out to the reg using some algorithm. I have flown in a Cessna 400 which was showing some N-reg numbers.
This works with the Avidyne TAS6xx boxes if they are configured for RS232 (not ARINC429) output, but the RS232 output works only with Avidyne's own MFDs, AFAIK. I have a TAS605 driving a KMD550 and a SN3500 so I don't see any regs.
For non-N-regs, the decode from the 24-bit ID to the reg is nontrivial; it needs a database lookup. I think these boxes do that - as well as much else e.g. triangulating the position of targets not emitting lat/long which means anybody anywhere can now have their very own "unofficial radar".
Sure, money can be a problem, but from what I have seen over 12 years I think the biggest problem is disputes within syndicates, whereby 1 member can block almost any expenditure that is not absolutely legally required.
If someone doesn't want anyone to know who they are and what they're up to, don't squawk. If you want to avail yourself of some kind of air traffic service, you'll have to tell the unit who you are and what you're doing - in which case the most capable transponder is surely the best thing to have?
Probably half of my operational hours are in a Mode C veil where I am required to squawk but not required to contact ATC. If Mode S were required (it isn't anywhere in the US under 18,000 ft altitude), I might be faced with the situation of ATC trying to control me because I'm flying towards their airspace, although not in it. I'm happy to skip that pleasure, and also the pleasure of having the Federal Government track my movements without reasonable cause and without my permission - the illegality of which is a very real concern, and why Mode S was never implemented in the US. The Feds are trying again with UAT and ADS-B out, we'll see how that goes.
Mode C plus ident works well if you need a transponder, and I guess for that reason I haven't seen an aircraft with Mode A only since I've been flying. I do vaguely remember some in the 70s, flying with my dad.
I think the Federal Government point is a very US one. Personally I don't get why it is such big deal.
Fly with Mode C and tell the world there is something there at X altitude.
