Also, on the subject of post-64 silver halves/dimes/quarters there are several commemorative ones (and dollars) that you occasionally see.
There are the 40% silver Bicentennial sets (Quarter, dollar, half) in both proof and business strike. They look like a shinier 40%er with a bright white rim, the other day I was lucky enough to have a teller show me a collection dump she got, she kept the 2 40% Ike dollars, the statue of liberty dollar commemorative and several uncirculated/proof clad halves (including some statue of liberty commemoratives) and she sold me 8 silver Roosevelt dimes and the 40% silver Bicentennial quarter because she didn't want them.
So yes, people do (for some reason) decide to crack open proof/commemorative sets and spend them for face value.