That was one of the reasons why I suggested Fett as an alternate source for the information.

Vader wants the Rebels in the Emperor's trap, he has previous form with Fett, while Solo and Leia may find it unpalatable the Rebels have already had dealings with shady people. As fan service it's functional.

Who give a damn about Bothans?

Outside the EU we have no idea who or what they are, for all we know they could be the Imperial equivalent to the Boy Scouts movement which explains Mon Mothma's portentous delivery of the line.

It was a cheap emotional button push.

Knowing that the Rebels are heading into disaster before they leave is much more suspenseful than having the Emperor surprise us all with it and then end up with egg on his face as he falls screaming down a well.