Great discussion, my thoughts:
Regarding resource usage: This isn't an issue at all. Having 1 or 10,000 users that have 0 posts won't really effect anything. What uses resources is the 100,000 pageviews per DAY and each one having about 10 database queries each... that's a resource user, but that's also why we've got a very robust server and constantly do upgrades.
Regarding deleting "inactive" members: I don't see any pros to this, especially considering the above. I'd rather have 1,000 inactive members than delete a user who just went away for a few months and came back to find their account deleted... that wouldn't be a very positive experience. Again, since no resources are used, I don't see a reason to delete them. Most forums I visit do NOT scrub out "inactive members".
Regarding sponsors: Yes, sponsors should look at forum members, but they also look pageviews (impressions), unique visitors, sessions (visits), clicks, conversions, active members online, # of threads/posts, etc. Again, "# of members" is relative, but since most forums do not scrub / erase inactive members it is a good relative measurement.
Regarding "cliques": Sure, with such a great community of friendly people you will find groups that gravitate together. Nobody should ever feel excluded from the forum just because they aren't active in any of these "groups". This forum is big and supportive enough for everyone.
We do have a newsletter that goes out to all the members and we're always encouraging everyone to be active with the community. We've got such a great group of people here that make this place such a fun, friendly, and safe place to be (thanks BYC Staff). With that said, we've all got multiple responsibilities and sometimes they get in the way and interfere with our addiction!