U.S. Letter size predates the European standard. The U.S. Letter size apparently originates with vellum sizes and was originally intended for handwriting not typography[1].

However, there are some good ways to use 8.5 x 11 [2] that might look odd on narrower and taller paper.

[1] http://marginalrevolution.com/marginalrevolution/2012/09/why...

[2] http://retinart.net/graphic-design/secret-law-of-page-harmon...