I can't answer the Mode S thing.....
But I would be inclined to fit some sort of txpdr, if for nothing else to help get zone transits and the like. I recently looked at a Permit aircraft, and although it is possible to get a class D zone transit without one at the moment, you could get refused one, and if the a/c is fairly slow, this could make a big difference. The pilot in my case got a D transit ok, but the radar controller couldn't see him all the way through, even with the gain turned way up (so he could "see seagulls")
The other time might be if you wanted to enter the Channel Islands Class A, which I would from where I live. I don't know their policy on allowing non-txpdr equipped a/c in, but if you've got one, it's going to be more difficult to refuse you entry......
Plus there is a definite safety aspect, its nice to know that controllers (and military controllers) know you're about, even if they aren't talking to you.
Just my view.