I actually had the same gripe and avoided closed mouths for talking characters in my first comic like the plague. I even had a discussion with my dialogue editor who wanted to add text to a character with closed mouth. How could he!?
Going to my current comic, I actually changed my opinion on it. It features plenty of panels where closed mouths speak.
A panel isn't just the moment of the character saying their line. It can also be before, after or in between. The character smiles, then says "...yeah, you're probably right". There is no need to show this in two panels - it can be done in one.