It seems like we need to keep the format at least at parity with use cases rather than behind.  Otherwise, people will implement their own custom encodings for things the format doesn't natively support.  This will in turn mean that files are not interoperable.  The cost of adding new ConvertedType's is small and we can deprecate things in the future if absolutely necessary.  I'm sure we'll get some things wrong the same way all projects do.  

If we want people to be able to move data between databases and parquet it would be good to support the most common types.  Virtually all the databases implemented these types because there are many use cases for them.