I still cannot understand, why should we solve a syntax issue by adding a semantic property.
We don't save any of microdata benefits (must write new parsers, new syntax logic etc) and make a strange hack.
Why should it be included in semantic schema? There can be other (more beautiful) ways to handle multiple types, like Adrian wrote or something else.
It's all is a Microdata syntax extension in fact, and using a semantic property for this purpose is an ugly way.
Let's just say we support another microdata-like syntax, it's much easier.
18.06.2012, 20:23, "Sandhaus, Evan" <sandhes@nytimes.com>:

My $0.02.

While I think that the ideal solution would be to extend Microdata Syntax to allow for multiple types, this solution will take time as it necessarily entails a standardization process. As an active member of a Standards Body, I certainly value and sympathize with the need for a deliberative standards process.

That being said, I feel that the need for web publishers to associate multiple types with their metadata is sufficiently urgent that the sponsors of schema.org would be completely justified in introducing an 'additionalType' property. Far better, in my view, to have a good solution tomorrow than a perfect solution at some indeterminate later date.

All the best,

Evan Sandhaus
Lead Architect, Semantic Plaforms
IPTC Representative
The New York Times Company

Egor Antonov