TL:DR River Polygons with river lines good, river lines alone bad.

Water Body digitization is the base line of water networks on openstreetmap. Every point, line, and polygon has been either manually created or generated through a computer application. Much like roads, rivers and other water bodies can be and sometimes are crucial resource for transporting goods.

Historically, cities and small towns were generally near a water body for drinking/bathing water, cargo transportation, and in the past one hundred and fifty years rivers are used for electric power generation.

Water Body Polygons are used in all types of maps so why not make these features correct by providing a more detailed polygon over just providing a line.

EX. The Sprague River in Klamath County Oregon is a perfect example of having many twists, turns, islands, and variable width across it’s path. Detailing these features in larger scales may not be as necessary as smaller scales below 1:150,000.