I think marketo should send an alert when we try to send an email that is not approved.
Let's say, you prepare your email, approve it, attach it to the flow "send email" to prepare your marketing activity.
And then you want to rework on your email. Your email is then in draft status. Your forget, and you run your marketing activity. The email that will be sent would be the last approved version, which is probably not the one you wanted to send.
Marketo should program an additional step which would warn the user "Be careful, your mail is in draft status, approve it before sending".