Should PSCD run in a separate instance?

I know this question was asked before, but they were only looking at the performance issue.

We hired a PSCD functional consultant to help us implement PSCD. But based on his passed experiences, he says that SAP recommends to install PSCD on a separate instance. Why? No idea? I cannot find any SAP doc on this.

This is why I turning the question to guy 😊

Thanks a lot for your help,

Eric