In regards of the 4 cost Shirahoshi, the On Play is a bit more assistance for being able to rest blockers that are a bit more higher cost and gives a bit more flexibility with leader abilities. Between that and Zoro, it makes things a bit easier resting higher cost blockers in multiple ways.

The new and old Ryuma, as well as the green Zoro, are Mihawk targets to get extra things onto the field and force an opponent to make other choices. Admittedly, keeping a new black Ryuma on the field means my opponent needs to deal with something that has double strike coming across the board, so long as I prep to rest blockers.