A big point for me is: maintenance. When all of the CSS is in external stylesheets, it is going to be relatively straightforward to find the styles your looking for to edit them. If there are inline styles, it will be harder to find what you're looking for.
I'm usually skeptical about claims that a style never be reused. If it happened once, it is likely to happen again. Coding for reusability is always a good idea and usually results in cleaner, better written code in my experience.
Even in these situations, I will usually add a unique class somewhere up in the target element's ancestors - often on the body tag - that I can use to target the the "unique situation" and keep all the CSS in the external stylesheet.
That being said, I do occasionally use inline styles. When I do, it is almost exclusively in a situation where I know that I will be animating styles with JavaScript after the page has loaded. I only do inline styles for the properties that I am going to animate and the rest go in the external stylesheet.