Backpacker - Glad you're getting some experience poling an aeroplane around properly! In my own opinion, the idea of a "bank angle limit", especially one as low as 20 degrees, is a thoroughly stupid idea. Yes, by all means, teach students that the turn onto finals should be "gentle", but imposing an artificial limit just introduces an extra distraction - sapping mental capacity by forcing them to monitor an extra parameter, and potentially introducing the problem you mentioned, whereby a student would sit at 20 deg AoB and try to kick the aircraft round the corner with rudder, rather than increasing speed slightly and manoevering the aircraft properly. It also seems to introduce a horrible mindset where people seem to think that the aeroplane will drop out of the sky if manoevered at anything above 30 deg at any flight conditions.
I personally would be an advocate of introducing low speed agressive manoeuvering as a compulsory part of the PPL (if not some mild aerobatics), just to give people some idea of where the limits of an aeroplane actually lie. Just my opinion!