First, I do think PowerBooks belong in their own category due to their unique issues. Separating 68K PowerBooks and PPC PowerBooks seems like a good idea as they too have very different issues and software considerations. However, if those who have a broad understanding of both may not visit two separate forums, thus limiting the spread of their expertise as widely as possible, then I think it's a bad idea. Though it's a unique case, splitting the forums would effectively orphan the Duo 2300 from the rest of the Duos, even though many of the things that apply to it also apply to the earlier 68K versions.
As far as limiting expertise to 68Ks:
1) As long as 68K software runs under a PPC, even if it is only under Classic emulation, then PPC Macs should be included.
2) I would guess a large percentage of the forum member use Intel-based Macs (or will use them) for their routine access to this forum and as such may need assistance with both the operation of those Macs and downloading software for use with vintage Macs. Not to mention 68K emulators such as Basilisk II and Sheepshaver.
3) Why send users all over the internet looking for information about PPC Macs, most considered vintage by Apple as well as current Intel Macs when there are other members who are equally helpful with those topics. Putting them in separate forums allows those who want to participate in those subjects to do it.
4) Eliminating nothing but 68K Macs would mean all Apple IIs. Let's face it, Vintage is Vintage. What attracts us to Apple products should not be limited to the processor. If server space were an issue I would suggest limiting it. Since that does not seem to be the case, there is no reason to make anything taboo. State the goals as 68K oriented forum, which should attract mostly those who have an interest in 68K Macs, but doesn't drive them away to another forum to discuss something else most of us are perfectly willing to indulge.
I mean sometimes you just get tired of eating Chinese.