OK, you can't steer with the rudder -- not directly, literally, technically. BUT you can use the rudder to induce a plane to do the things that result in a turn... and that will feel just like steering for most people, especially for a new guy. And when a plane's going too slow to use your ailerons, it may be a good thing to do. When you have to tweak the heading or attitude of a 4-channel plane that's just above stall speed, a good healthy aileron input may just snap your plane right into the ground. (Granted, the original question was about turning onto final where ailerons should be used, but it's the "never, never" thing I wanted to clarify, not the final turn thing.)

We also want to line up straight on the runway as early as possible on final approach, but the wind does blow... and for anyone who thinks that the wind ever comes in from a uniform direction or at a uniform speed, a day at a weather station would be a real eye-opener. A plane on final may need directional inputs right up to touchdown, and for a 3-point landing that means right down to stall speed.

I agree that ailerons are the preferred way to steer (or maybe the ONLY way, technically, to steer), but the only reason we can credibly tell people not to steer a plane using the rudder is because they are indeed able to give a rudder command to directly or indirectly cause their plane change course or heading... and there are times when that's how it has to be done.