I may be a bit of an oddball, but I don't think any position is "required". I try to make certain no scenario I build has a single-role lock. Lacking a role can make the scenario more challenging, it may remove some of the flourishes, but it is never a requirement.
That said, you usually need a matrix spec, a magic spec, a combat spec, an investigator spec, a socializer, and a transporter. A sneak is also a commonly requested role. A player can fill more than one role, and more than one player can contribute to most of the roles (Matrix being the usual but not total exception.)