Hi Mike,
It's no coincidence the relative abundances of H, He, O, N and S decreases as the number of protons increases in the nucleus. This is due to the nuclear binding energy decreasing as one goes from O to S.
The more protons in a nucleus the greater amount of energy required for nuclear fusion to overcome the Coulomb barrier due to the electrostatic repulsion of the protons.
The other reason why S for example is rare is that there is no direct fusion process. The formation of S nuclei is through a complicated alpha process requiring a number of preliminary steps for fusion amongst lighter nuclei.
The more complicated the fusion process the probability of the formation of the end product is decreased.
Hence SII emissions are faint simply because there are fewer S atoms.
On an imaging front SIIRGB imaging is comparatively rare but potentially rewarding.
Since the SII filter passes longer wavelengths than Ha, OIII and NII, filters, it can potentially penetrate deeper into planetary and emission nebulae.
Here is a SIIRGB of the Eta Carina nebula. The structure surrounding Eta is enhanced when compared to HaRGB images.
http://members.iinet.net.au/~sjastro/etaSIIc.html
Regards
Steven