Thanks Manish...
Looking through this note it says that Maintenance Certificates can be installed via two methods, one is via SolMan the other is without:
Manual (without using Solution Manager):
In this method you request the maintenance certificate via the License portal and receive it as a text file. Then using this text file via transaction SLICENSE you install the Maintenance certificate on the system. A detailed Procedure can be found here: Manual Implementation.
So we DON'T need SolMan to do add-ons/SPs as we can manually apply the certificate.
Is there any other reason/legal requirement we'd need SolMan?