Startup CEO Coach, Culture Development Consultant, Entrepreneur, Startup Investor/Advisor
Here is the most useful article I've seen on the subject: http://techcrunch.com/2011/09/22/free-startup-docs-how-much-equity-should-advisors-get/
It's important to consider the stage you're at and the level of involvement of the advisor.
Hope that helps!
I was on a panel on startup culture a few months ago with John Lilly, former CEO of Mozilla, who shared his view that culture building is part poetry, part pragmatism. The poetry part would say only hire/keep people who share your values and fit the culture. Period. This is the gospel being preached by companies like Zappos, Southwest (and often, me :).
The pragmatism part would say that if there's someone with a unique set of skills that's truly hard to find, it may be worth keeping them on if you can minimize the impact on the culture.
Most of the time, people err way too far on the side of (perceived) pragmatism and keep people who don't fit the culture for too long because they underestimate the negative impact on the team of keeping someone who is a bad fit and the opportunity cost of not having someone who's a great fit, and overestimate the cost of losing that person (loss aversion). I repeatedly encounter situations with my startup CEO clients where they're scared to let someone go, and once they do, performance actually improves overall (even before they've hired a replacement).