As for everything else, I have ran into situations, like “converting” a Photoshop mockup aka PSD into code, and every web page having unique css styling and positioning that is never used again on the other pages and almost guaranteed little to no modifications in the future. Instead of flooding the main styling sheet with one time use styles and then having to flip back and forth to which page or file is using that style, in that case, I have found that it is much easier and convenient to have the in-line styles, though it still should be avoided. Flipping back and forth or scrolling down a long list of unreadable css classes just makes it harder to modify. We all use external CSS stylesheets in order to make life easier.
There are ways to reduce the need to have in-line CSS styles. One is repetition, which is something you probably learned from web design or design class. Reusing the same styles as much as possible and keeping design as consistent as possible, makes the individual web site pages more unison with the whole site.
So as much as I value web standards, I feel some rules have to be broken to have cleaner code and easier modification. However, if you find that you need a massive and excess amount of in-line stylesheets, then there may be something in your design that should be improved. Moderation is the key.