You wont require SQL license to backup Sharepoint you will require only sharepoint license...........If you are backing up SQL instances seperately (not under sharepoint) then you will require SQL license.....http://www.symantec.com/docs/HOWTO73808
You are correct, the sharepoint agent also includes the right to backup the sharepoint SQL databases.
But if you also have other databases (or other database servers) you will have to purchase the SQL agent in order to be complient to licensing requirements.
.If you are backing up SQL instances seperately (not under sharepoin
If you do this, then when you restore the SQL instances, you might find that you have a logically inconsistent Sharepoint farm.