Unit Test Project Structure

I recently came across Phil Haack's interesting blog post and associated poll about structuring your unit test projects - do you have separate unit test assemblies or not? - and was not entirely surprised by the results.  They were more or less in line what what I'd expect.  At the beginning of my TDD "career" I'd actually considered for a brief second whether it'd be better to put my unit tests in line with the code they're testing (not "inline" as in the same file, but close) or in a completely separate assembly.  After a few minutes of thought, I came away with what it seems most other people come away with - and emphatic "NO!"  I'm honestly left wondering - and I don't mean any disdain, but - what are these 8% of "Yes" voters are thinking!?  Is it laziness, or some other motive that I'm not getting?  Really - I'd love to know!