SSL VPN - How important is certificate?

Dear forum,

This is our current SSL VPN Security checks:

- RADIUS check for user/password. - Certification check.

- FortiToken check.

The IT team and I are discussing the need for our end users to continue using a certificate, to authenticate for our SSL VPN solution. We have recently deployed Two-Factor Authentication, for this reason I presented the idea to stop using a cert, since it give us a lot of support issues from time to time. People often had corrupt certs (error 5) and we had to reinstall the cert to make it work again..

My question is, how important is the certificate authentication security wise? Traffic from FortiClient to FortiGate is still SSL/TLS encrypted without cert, when people type in username/password and are prompted for a FortiToken, right..? The Certification is just "another check" and has nothing to do with the traffic? Or am I wrong?

Any input is much appreciated!