I don't know that I'd say that all sweaters must be left out. It depends on the sweater. For instance, I have a couple light-weight, body-hugging (more or less), turtleneck sweaters in wool/wool blends that I prefer to wear tucked in with a belt. But a bulky jumper or a sweater-vest I would probably wear on the outside. Whether or not I wore a belt with that would depend on whether I thought I might take the sweater off at some point during the day.

Regards,
Rex.