Morten Rand-Hendricksen on how ambiguity in Post Format in WordPress makes life difficult for users and trainers…
This example outlines three wildly different user scenarios, all of which are more or less mutually exclusive:
An image post should have one image, and one image only.
An image post should have one or more images where the first image is considered the image.
An image post should have one URL pointing to an image and the theme will wrap this URL in an tag and use the post title as the title attribute.
There are also many unanswered questions: Can an Image post have text content? Should the more tag be used? What content should be displayed in index pages and in the single post template?
But the biggest issue is what this ambiguity does to consistency: If you use a theme that employs interpretation 3 and then switch to one that employs interpretation 2 or 1 you will have to reformat your content or end up with a sub-optimal user experience for the visitor (in this case providing an unlinked URL in place of an image). The same goes for other combinations, and this extends to all the different Post Formats.
From a design and development perspective this may seem like an attractive lazzis-faire approach, but for the user it is a setup for chaos. When the designer and developer are left to interpret the Post Formats as they please the end result is that the user cannot expect consistent behavior across themes when using the feature. Which is an excellent reason not to use the feature at all.