What does it mean that a site "eats up needless room?" — In an academic sense, it would be nice to see everyone collaborating on the world's religions… but those you listed are not academic sites.
They are not interchangeable subjects you might pick and choose between based on the context of your problem (like choosing a programming language on Stack Overflow). The members of these communities aren't interested in the local catch-all "religion expert" to weigh in on their deeply-held religious beliefs. These are applied sites, not tags under some academic "world religion" study.
Sites and their subjects are formed around communities; communities with common goals, problems, and interests. The Christians and Jews on those sites want to consult and commune with those who are of the same teachings and belief system.
In the blog post Merging Season, Joel came up with a set of rules to see if is site X should be subsumed by site Y. So let's substitute "Christianity" for (X) and "Judaism" for (Y), and see if this marriage of subjects sounds like a good match.
Here’s the best we could come up with for deciding whether X [Christianity] should be subsumed by Y [Judaism]:
- Almost all Christianity questions are on-topic for Judaism.
- If [a subject for] Judaism already exists, it already has a tag for Christianity, and nobody is complaining
- You’re not creating such a big group that you don’t have enough experts to answer all possible questions
- There’s a high probability that users of Judaism would enjoy seeing the occasional question about Christianity.
The missive above doesn't sound workable to me, but whether you agree or not with that technical assessment, there is still one incontrovertible problem: Users seeking answers to their deepest "expert" questions on Judaism (or Christianity) are not going to show up on a generic "religion" forum — And site is not much good to a group of users if they will not show up.