Smajdalf: Where exactly do you see simplification? I see 4 pin device + current limiting resistor vs. 3 pin device + current limiting device. In given context optocoupler is more expensive device with (at least) one leg to solder and no extra gain.
One single 16pin DIP device, lower profile, [u]compact siz[/u]e. They are even available in [u]SMD[/u] packages. One component to fit instead of 4 or 3, legs pre-aligned to 0.1inch pitch. Limiting device in both concepts. Thats FIVE components for 4 buttons. OR FOUR components for 3 buttons. Tom.... :)