When I embed an object from another site - e.g. a YouTube video or a Google map - into a blog post within the visual editor, the embed code often gets corrupted. This happens most especially when I attempt to edit a post using the visual editor in cases where an object had been previously embedded in that post.
I have no problem using the HTML editor, but sometimes I open a post in the visual editor accidentally and it screws up the embed code.
Also, for some blogs I collaborate with non-technical people, and they don't ever use the HTML editor.