What attracted me to StructuredText in the first place was the fact that it's the only structured document format that I could UseWithoutThinking?. No matter how much of a DocBook wizard or HTML wizard you are, you still have to constantly think about elements, entities, and what not. By contrast, you can largely use StructuredText (at least, a bug-free version) and concentrate on your text rather than your formatting.
Obviously this makes it very easy to use for experienced authors, but it also has tremendous benefits for folks who want to use it as part of a content delegation solution. StructuredText can be picked up in a very short time by non-techies and is probably the only solution available where people who don't necessarily think about document structure can author documents that are semantically parsable (as opposed to Microsoft Word garbage).