The basic motivation here is that I'm remodeling all three bathrooms because structurally, code-wise, functionally and aesthetically they are a disaster. The problems with the status quo are a litany and I don't really want to bore anyone with them, but suffice it to say that, by the time I removed the things I had to remove, it was evident that I would have to take it down to bare framing because there just wasn't enough left to try to preserve. This is mainly attributable to a previous "handyman" whose traces I'm doing my best to eradicate. In so doing, I'm hoping to "do things right" and not be another in the line of boneheaded handymen to plague what is essentially a beautifully built house.
The manner in which my house is plumbed doesn't have any venting besides the main stack, so I'm not sure that issues with a combined drain and vent are why it's not currently in use. Although I'm sure that the last remodel (by the "handyman") was in the mid-90s, by which point codes had long required this, none of the fixtures are vented. So sloth and lack of giving a hoot are a more likely explanation for the absence of any particular type of venting, and all venting in general.