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IMO there's only three things Google can use to tell what's in your 'niche'. Backlinks/neighbourhood is the first. Second is page content. Third is site content.
Beyond that, I suspect that they do a pretty crappy job right now of defining niches. A boating site might do well with links from fishing sites, but how anyone would define that they're in related niches is beyond me. I suspect they can't really.
Worse, some authority sites can't be placed in a niche. A link from a professor's page on a university's site? A link from wikipedia? Good links (disregarding nofollow) and likely to be solid for SEO purposes, but not exactly from sites in your niche.
In other words, I suspect such links are just fine, and doubt that they'll be getting anywhere with it anytime soon. Nevertheless, if they *could* start defining niches, you're going to get clobbered. Thus why I wouldn't suggest it as a good method. A small amount, sure, more than that, no.