The Simmons drum pads eventually contributed to the demise of the popularity of the kit. The first generation pads featured a hard plastic playing surface, which was advertised at the time to be the same perspex that police riot shields were made of. It led to claims of wrist tendon injuries by some players, and it had a loud clacky sound.
The second generation of pads had the same cool external shape as the originals, but had a softer rubber playing surface. Somewhat softer, but there was a trade off in trigger sensitivity. The second generation pads were just a thin piece of rubber on top of a plywood board. The middle of the plywood board had a hole in it, in which was mounted via silicone caulk a cheap piezo transducer.
It was a vast playing improvement in comfort, however, over the harder original pads. It would be interesting to see with some of the high tech rubber compounds we now have just how comfortable a Simmons pad could be with a better covering.
In any event, when I played a hybrid setup, with the Simmons toms and snare and an acoustic bass drum and snare, one big advantage was the slim nature of the tom pads. The tom pads could be placed very low over the bass drum, meaning more dexterity for me as a drummer. Often times, I played a basic 4 piece acoustic kit with a couple of Simmons tom pads over the acoustic toms and I'd put the Simmons snare pad to the left of the high hat for some variety in sounds.
Many drummers were doing this long before I was. Many studio recordings had similar setups, and often had the acoustic bass drum and snare also triggering the respective electronic sounds. Many touring bands, the big ones, augmented their drum kits with a few electronic pads to again add some different sounds. As technology improved, these bands with lots of equipment bucks could trigger edrums sounds off of the microphone impulses of the acoustic drums, thus removing the electronic drum pads from the stage.