In the 18th century, when the big belted plaid got seperated into two components - the little kilt and the (now smaller) plaid - the latter was still carried afield by the Highlander for use as a blanket at night. He carried it over the shoulder and wrapped around his body, much in the same way that Confederate soldiers in the Civil War carried their blanket rolls.
Modern pipers and drummers are merely imitating that "look" with their plaids....
Brian
"They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety." ~ Benjamin Franklin