No, it is not necessary for all of your participants to join via video in order to access your conference. If some of your participants don't have a webcam or don't have access to the Internet, they can still join your conference.
Those joining your conference online can access the call in exactly the same way. The only difference will be that there will be no video available for that participant. Download the 'Participant Guide' here for more details.
Joining By Phone
Callers using their phone can call in with your regular Dial-In Number and Access Code.