It's true, I don't have any problem with using SPACE via left thumb, but I have never encountered a layout or keyboard that let's me use the thumb for modifier keys. Do you have an example?
The thumb (and thumb knuckle) keys are my favorite design point, but the entire design is excellent. They are out of stock, but the designs are open source, and there are a lot of other great DIY designs around.
The Ergodox has thumb clusters similar to the Kinesis Advantage. There is also the Dactyl, and many many more.