Probably much less than 1% English speakers know how to to select a text encoding in a browser, but for speakers of languages where multiple non-utf encodings were common until recently this knowledge is more common.
My anecdata: once having only a smartphone with me I needed information from a txt file in cp1251 encoding. The server was configured correctly for .html, but not for .txt Mobile browsers available on the smartphone didn't allow me to override text encoding. It was very frustrating experience and it would be enough for me to switch to another browser in future except I don't know a mobile browser which allows to select text encoding and not much worse than mobile Firefox. And I don't blame the maintainer of that site - he does (or did) this in his spare time and had more important tasks than to check text encodings for every single URL on the site so users of crippled web browsers would have no problems.
I like comparison of this feature to emergency services - you rarely need it but when you need it you need it very much.
My anecdata: once having only a smartphone with me I needed information from a txt file in cp1251 encoding. The server was configured correctly for .html, but not for .txt Mobile browsers available on the smartphone didn't allow me to override text encoding. It was very frustrating experience and it would be enough for me to switch to another browser in future except I don't know a mobile browser which allows to select text encoding and not much worse than mobile Firefox. And I don't blame the maintainer of that site - he does (or did) this in his spare time and had more important tasks than to check text encodings for every single URL on the site so users of crippled web browsers would have no problems.
I like comparison of this feature to emergency services - you rarely need it but when you need it you need it very much.