Seth Godin's blog: 

Writing a spec is a kind of mind reading, which is why it’s so difficult. One half of the partnership has to take the time to not only specifically and precisely write down what’s expected and what the measurements and boundaries are, but then must do the challenging and risky work of engaging with the other half of the team to agree on that spec. Disagreements here are cheap, disagreements later cost a fortune.

The fear, of course, is that the spec will end the project, that without a lot of sunk costs on the table, a spec alone is too easy to renege on. In my experience, the most successful freelancers are also the most successful spec writers. Yes, there's some risk in clearly and vividly making your promises while the client/partner/boss still has time to back out. But professionals take that risk every day.