This results in less drivetrain movement without the need to use higher durometer rubber. Rubber also serves as a more effective damper than polyurethane, as it is able to tolerate load in extension, in compression, and in shear. Polyurethane is really only effective in compression.
Rubber also does not pack and wear like polyurethane does, so you'll wind up with a longer-lasting and more consistently performing part.
The BSH mount is just an insert, and still allows for a significant amount of movement in the stock rubber mount before limiting movement significantly.