1) Why don't you provide spectra with nebular emission lines?
Simply put, the uncertainties in gas properties in galaxies are comparable to those in the stellar models. We encourage you to explore your own gas parameters using a radiative transfer code such as MAPPINGS or CLOUDY. See the Emission Lines page.
2) How do your results differ from those of other codes?
It depends what you're looking at! These models are based on a set of detailed stellar evolution models and include prescriptions for binary mass transfer, rotational mixing, supernova kicks.
For these and many other details, please see the BPASS user manual
JJ gave a lecture and workshop using BPASS models at the ANITA summer school in 2015. Lecture notes and instructions can be found here.
Both JJ and Elizabeth are happy to present our work at a variety of levels, from school level outreach to scientific conferences