Sure, I understand that this is not a requirement that everyone will have, but when you do need it, the behavior is not what you expect or likely need. I'm happy to share some more detail since you're interested. If you look at the below screenshot, you can see some of the options that are really only possible in a transport rule:
You can get very creative with some of these use cases, but just take message approval for example. Let's say you have an overly zealous owner of a startup that want's to approve every message sent that includes an external (outside of the organization) recipient. An employee sends out a message that is addressed to both internal and external recipients. In the current rule options you would say "require approval for any email to a recipient outside of the organization". The problem is that all internal recipients will still get this message even if the owner decides to reject the message for external recipients. So now you have a situation where the internal recipients received the message, but the external recipients did not. There is no way to say "It's ok if the message is only sent to internal recipients, but as soon as an external recipient(s) is added along with internal recipient(s), I want to approve or reject the entire message". That might be a terrible example, but I think it helps explain what we are missing here.
Our specific example is that we use a product called MailApprove (mailapprove.com) which requires users to self-approve emails that match certain criteria. The product works great, but the flow logic requires some internal/external recipient messages to be processed (and bypassed) unnecessarily.