I guess your question is whether you need a cover letter even if you are working/have a referral.
The answer is yes, you should go for it.
Think that, if its not the CL, you will need to write the same text for the referrer in a mail or so, so it´s a text that is indeed handy to have.
This said, it´s more the content -that then you will be able to leverage in multiple places- what matters most, since the CL by itself is not a very important document.
Hope it helps!