Board members and (good) investors are always de facto advisors. And if you seem promising, people in the startup world will help you out for free. But it is not a good idea to have formal advisors who are neither board members nor investors.
Lots of great companies have formal advisors who are neither board members nor investors. I encourage entrepreneurs to turn proven smart + helpful people into advisors.
The board's primary task is governance, not advice. Lots of great advisors don't even want to deal with being on your board. pg is probably a good example of that.
I would also argue that, to some degree, YC is effectively acting as an advisor, not an investor. This is consistent with the fact that some/many? of the YC grads say that they would give YC the equity for free, just to participate in the program.
I'll write up detailed thoughts on Venture Hacks soon. I'm sure you're holding your breath. =)
Stock is worthless anyways, why not give out a small amount of equity to people who help you out? Sure, lots of startups don't have advisory boards, but what's the harm in having stock-holding advisors?