In combat, it's about facilitating speed of play over being forced to stay perpetually in character. Maybe the liesmith is asked if they can handle a certain enemy. Maybe the cleric is asking who needs healing. Maybe they have to tell an ally their plans. Maybe they have to express their tactical condition. I'm sure it's possible to sit down here and think up perfect lines for any individual situation, but I'd rather not put combat on hold while the player thinks up just the right wording. It's about keeping things flowing at a decent pace.

In short term tactics, the liesmith's skill list lines up with a beguiler, bard, or character with a rogue level or two. Imagine the character scouts/asks their familiar to scout, and then reports back to the party. It's easy to say "I didn't see four ogres who weren't really paying attention", but that's the sort of thing a kooky gag character does. Trickster archetypes are not kooky gag characters.

Long term, good cons work best with an accomplice. Silver Tongue basically guarantees you're the face/diplomancer/consummate bull**** artist. Reporting back to the party, setting up plans, and possibly coordinating the social game. These are things where players usually speak plainly to get back to the scenes ASAP. Either the player is forced to slow these segments down to pick their words carefully, or everybody handwaves the drawback away on their own. See the comment about mute characters. Since it's bound to happen anyways for ease of tabletalk, you might as well acknowledge it.