Could be quite practical for a soldier. Looking at an article in the Sealed Knot knowledge base (a great resource for colgaffneyis and friends, BTW) I found a description of Prince Rupert at the Battle of Marston Moor in 1644.

“He never wears a Beaver in the field but a Dutch White hat, which being to ingage himselfe in fight he pluckes down about his eares as low as possibly he can , that paradventure he may loose but he can never loose his steele cap, for when he puts on that, he takes up his haire (which he weareth very long) and throwes it quite over his cap, and that it might not fall down againe he ties it close above his cap with a silke Ribband so that he can never lose his Cap unlesse he doth loose his head with it which he was never more like to doe then now, for wore you what.”

His concern for protection was understandable. After the battle (which he lost), he was “covered with bloud like a Butcher…”