Forcing them to split the witness elements across many transactions increases the cost of spamming the network. That would mean fewer people do it. It also means more money for miners. The biggest win would be that it would effectively kill the image/audio/video inscriptions. Only the text based inscriptions would be practical after being split. But ultimately the point is to send a message to them that non monetary transactions aren't welcome. But instead Core chose to open the doors for them.