There are a lot of questions that get closed as "belongs on uservoice" now "belongs on meta" that would simply not exist if the search results and Related Question hints that show up when you ask a question simply included the meta questions as well. Many of them are legitimate questions about the use or etiquette of SO. New users aren't familiar with meta. . While the "feedback always welcome" link is inviting for offering feedback, its not obvious that it's the location for asking questions about the site.
Naw, let's not do that.
If we show meta search results in with SO search results, folks will just stay on SO for their meta-SO tasks, including asking more meta-SO questions on SO proper. Which kinda defeats the whole purpose of having a meta-SO.
Close 'em, with prejudice, and send 'em this way - then they can stay, if that's what they really want...
I disagree that meta questions should be included on the sites.
Anyway, cross-site related queries is far beyond the scope of anything we'll be able to do anytime soon.