g1smd - 10:07 am on Feb 13, 2013 (gmt 0)
You only need to test THE_REQUEST when you are looking to redirect a request. There is one particular scenario to look out for. This is where you redirect a request with parameters to a "friendly" URL and that request is then rewritten internally back to a form with parameters.
If you are not testing THE_REQUEST in your redirecting rule, once the internal rewrite has occurred in a later rule, htaccess is parsed again and this means the request will match the redirecting rule and be redirected again. This exposes the recently rewritten path back out on to the web as a new URL. That request is then likely to match your parameter-to-friendly redirecting rule again, and you now have an infinite loop.
Using THE_REQUEST does make for slighly more complicated code. THE_REQUEST is the literal:
GET /thispage?some=parameters HTTP/1.1
request sent by the browser. The RegEx pattern needed to match is obviously a bit more complex.