Here's a beautiful picture on that. The infantry sledge idea was used by almost every country, it turned out in some topic, in different ways and for different purposes.
Imagine an AT round struck this tank from the side. That would cause a serious loss of both tanks and infantry.
The lack of armor protection on Japanese tanks is due largely to the offensive mindset of the Imperial Army General Staff, IMHO. They hated (passionately!) anything that smacked of defensive applications for anything they used, no matter how useful or necessary such an application might be. This attitude cost them a lot in WW2.