In Confluence 5.8.x inline macros properly render inline regardless of what you want to show. However in 5.9.1-pluginsfour018 the output is unpredictable:
The macro is properly set to:
@Override
public OutputType getOutputType()

When the execute method of the macro returns (with xhtmlUtils being an instance of XhtmlContent):
return xhtmlUtils.convertStorageToView("<ac:structured-macro ac:name=\"cheese\"></ac:structured-macro>Hello", conversionContext);
the output is inline. However the following
return xhtmlUtils.convertStorageToView("<ac:structured-macro ac:name=\"anchor\"><ac:parameter ac:name=\"\">some_anchor</ac:parameter></ac:structured-macro>Hello",conversionContext);
it is not inline. In 5.9.1-pluginsfour018 the inline functionality seems to be dependent on what the macro renders.

Steps to reproduce

1) Build a simple plugin with one Macro in it.
2) Set the appropriate functions to return BodyType.NONE and OutputType.INLINE.
3) Let the execute method return the strings from my last comment.
4) The string with the cheese macro renders inline, the other one doesn't.