The SID is superior to YM2149 just like the Amiga 500 is superior to Atari ST... but still i don't mind YM-music.
"how come the ST users wanted to imitate the SID sounds?"
I think it's more that they wanted to evolve the sound, like the SID music scene evolved and got digidrums for example.
the "sid-voice" and "sync-buzzer" effects was not created to imitate the SID, they where made because it was possible to make them at all. I think the Atari ST music scene evolved great and taking advantage of the fact that the 4-bit soundchip is in a 16-bit computer
Because the sid-voice and sync-buzzer is only fake YM2149 sounds, they are not in the YM2149 specifikations... the effects are made by the Atari ST.
Spectrum and amstrad can't make these effects, at least i think so... not really sure.