I'll own that I erred when I posted that example extracting the $_REQUEST SG. However:
Consider the case of a nice idempotent form, perhaps it does some site navigation or preference setting, and it's a perfect candidate for a GET request. But for what ever reason you prefer to use POST with the form. Perhaps you kinda like the URLs simplified. But you also want, on occasion, to communicate with the form with a link.
And further, let's say you want to save one or more of those features in a cookie and use that value unless overridden by the form.
$_REQUEST is, in those cases, just what the doctor ordered.