I am creating a race (either near-alien or a human sub-species) in a science fiction setting that have an unfortunate "tell." While they can stay silent in non-social (stealth-requiring) situations, they are unable to stop themselves in social situations from constantly saying exactly what they are thinking. They will even do this when they are alone with themselves and don't think they are in danger or anyone else is around. If using sign language, they mutter with their fingers.
This makes it difficult but not impossible to lie or even avoid telegraphing intentions. It's even difficult for captured individuals to keep secrets from interrogators. They aren't as a rule nice, and aren't making much effort to get along with others.
This is NOT created to be a weakness, but somehow is either an advantage or side-effect of their genetic engineering. The engineers were not concerned about their creations rebelling, but were actively creating the race as their replacements.
What is the justification for my muttering super-soldiers? Best answers will explain how this behavior is actually an advantage.