The intent here is to make 2dsphere indexes behave the same as normal indexes with regard to sparseness. In SERVER-9639 the behavior was changed to mimic 2d indexes, which treat the index as sparse if a doc is missing geo fields.

Sparse-by-default means the query optimizer can't use compound geo indexes to satisfy non-geo queries.

Need to confirm that 2dsphere honors the sparse designation if it's chosen specifically and that it's possible to index a document with an undefined or null geo field.