core.internals checking `hasattr(item, 'reindex_axis')`

[jbrockmendel]

Block and SparseBlock each have a reindex_axis method that is not used anywhere, but a bunch of things in core.internals do a check of the form

if hasattr(new, 'reindex_axis'):
    new = new.values

if hasattr(mask, 'reindex_axis'):
     mask = mask.values

It would be nice to get rid of the unused methods. Any idea what this ducktype checks are actually intending to catch?