Actually, I don't think so.When an actor signs on to be a main cast member in a Trek show, are they also obligated to also make guest appearences at Cons.?
No, they are not. That's not part of the contract. That's an added perk that some don't consider a perk at all.
For example, I wish Kate and Jeri did more cons. They both always cite conflicting schedules, but I'm pretty sure it's just because they find them a little on the grueling side.