I'm of a split mind about this splitter in the sense that I'm not sure how useful it is - if your input is an optimized index then it has just 1 segment, so this tool won't be able to split it, right?
AFAIK a similar functionality can be implemented also using two other methods that would work on indexes with any number of segments: one method is trivial, based on a "delete/IndexWriter.addIndexes/undeletAll" loop that requires multiple passes over input data, the other would use the same method as SegmentMerger uses, i.e. working with FieldsWriter, FormatPostings*Consumer, TermVectorsWriter, etc. for a single-pass splitting.
So I guess I'm -0 on this index splitting method, because I think we can do it better.