Well, for one thing FS and Homeworld have quite a bit in common, while Quake has little to do with anything, and for another both have smallish, specialized modding communities while Quake has, on my last estimate, more fanboys than there are people alive at this moment, and could do without a new modding community. HW has one, but it's not much of one, and we could probably bring something to that.
And we hardly need worry about spreading ourselves thinly. The official FS forums are effectively dead (or really dead now, I guess), so we don't have much of a recruiting pool- however, if we go HomeWorld, we can attract existing members from that community to bulk of the ranks of the HW modders. And, failing that, little or nothing of the FS community will be lost anyway- nobody already here is gonna leave because we have or don't have HW forums. As of right now, we're a smallish community supporting modding for a mildly popular but quite dead game- little else. HLP is getting old, despite the hope of user FS3 in a year or so, and expanding our focus to multiple aging games could only benefit us- besides, what happens when we run out of things to do in FreeSpace?
All in all, I think we have more pressing issues (such as actually getting a mod archive online), but since adding a HW community won't slow any of that down, really, and we really don't have any prospects of anything new and exciting happening for a good while. At the very least, this will be an interesting experiment. At best, HLP comes out better than ever, with twice the user base and twice the experience and equipment. Sounds good, no?