Mainly in response to Nick's answer:
Don't forget, shopping questions (i.e. what shield should I buy) are off topic on any SE site, with some beef up however that question would be fine on this site.
Also, The quality of users you describe and the questions they would also be poor for any SE site, Arduino, EP&D, or otherwise. You want the core site to be composed of experts, with novices and beginners being drawn to the site. There's no point in having a separate site to house poor content (that appears to be your argument). From an SE perspective (Take SO for example), the benefit to having a single EP&D site (RPi and Arduino included) is better advertising. Also, everyone would arguably benefit from an EP&D site where everything EP&D was included. Ideally it would be the EE site that had a question scope that extended to firmware and/or kernel drivers/modules. RPi users think they're really special until it comes time to hook up those GPIOs to something useful. Sure you could buy an out of the box kit that does all the work for you, or use python, but where is the fun in that :-) ?? project questions like that are prime for an EP&D site.
In general though, it seems a lot of cliques just want their own 'board', but don't really understand what goes into making a site, and also don't understand why common interests should be merged into a single site. SO is really a shining example: There is pretty much every programming language under the sun in scope on SO, both compiled and interpreted languages. There isn't a separate site for python programmers, or windows programmers vs linux programmers, scripters, or arduino programmers, etc. If it's a programming question it goes on SO. C or C++ has a large enough following to warrant its own site, but that doesn't mean it should, for way too many reasons to list. Suffice it to say that
Common interests should stick together - Programming concepts are fundamentally the same across all languages, even if syntax is different. Thus, someone with expertise in a different language could answer your question, but may never even see your question if it's on some niche site.
A broader question scope draws more traffic - more traffic equals more Q&A and more search engine results, which equals better ad $, which equals the possibility of a sustained site and not a killed beta.
I'm with SF., a lot of us (myself included sometimes) just don't use the tools properly, and feel like every question should be of our own interests, which really isn't the case. Or, in the case of an RPi, feel like they should be able to ask every question about their project on a single site, which shouldn't really matter either. If I'm building a desktop, some of my questions (motherboard, CPU install, etc) would be on topic @ superuser, but when it came down to installing linux modules for my webcam, or the right driver to use, that might be a better question for the linux site, or Ubuntu site if I'm using that distro.