Personally, I think that's a very odd guideline. I can see why they'd not want suggestions to come with completely new sheriff / investigator descriptions, but those descriptions have a strong impact on any role's effect on the metagame. For that reason, it seems perfectly fine for role suggestions to propose an existing invest/sheriff message to go along with it.

In the end, DR is free to change anything about a role suggestion as he sees fit. That's not limited to sheriff/invest messages.