As americanau diophile indicated, SPDIF is a digital signal and cannot be use to a regular amplifier without a decoder. The SPDIF cable can carry up to 8 channels depending on the encoded signal.
Some amplified speakers has built-in decoding chips and therefore it can be connected using just one cable (coax or optical). Best uses of SPDIF is when watching movie or listening to music. Some games does not support multichannel through SPDIF therefore you will only get 2 channel signals. New type of coding called Dolby Digital Live may soon change that, as long as the programmers incorporate it on their design.
If you think about it the connection is only a device to transfer the signal from one end to another. You are less likely to lose much of the frequency through SPDIF than analog.