Yamato still is a great source of Hashirama Cells, since he basically is a weaker version of the first (But stronger than Zetsu)
Hashirama cells are very useful in many ways so... he could want that?
Oro had a zetsu inside of him. Zetsu can match anyones chakra. Yamato was an experiment out of a hundred kids and only he lived. Zetsu is a better way to go. Orochimaru can really clone zetsu I'm sure.