First of all, yes, all mentioned systems have been tested in space in unmanned flights as you have noted. Some of those flights carried animals (e.g. Mercury-Atlas 5 and Mercury-Redstone 2) and a few experiments on e.g. life support.
On the contrary, nothing has actually been build which could match Bion or Zond spacecrafts. As of today, those Soviet / Russian programs are still fairly unique.
There were a number of proposals for "spin-offs" of the mentioned programs. Gemini saw a few ideas towards an advanced transportation craft and, most notable, an intensive program on potential military use. For Apollo, there was the Apollo Applications Program. There were tons of concepts, e.g. even one for a manned Venus-Flyby with Apollo technology. In the end, Skylab became the only actual "spin-off" of the Apollo program. Politically, the focus was directed on manned space flight, thus astronauts supervising experiments in space.