I believe it used to be recommended that applications accept ; to separate query string parameters so that if a url containing query string parameters was double-html-escaped for some reason...

e.g. a link to

  http://example.com/?colour=blue&age=old

with link text

  balloon

should appear in html as

  <a href="http://example.com/?colour=blue&amp;age=old">balloon</a>

but might erroneously appear as

  <a href="http://example.com/?colour=blue&amp;amp;age=old">balloon</a>

...or if a (simple) client neglected to html-decode the uri for some reason, then the link would still work.

Whether this was officially recommended or was merely a folk recommendation, I don't know.