One telling point is missing in the whole "turnback" discussion. Practicing it is just not the same as a real failure because you know the engine is going to fail and the "Oh ****" moment that happens with the unexpected real failure almost invariably results in a slower and initially less than optimal response.
Thus I think there is a danger of unrealistic feeling of attained competence in this manoever. I think it is also important to point out the powerful ground rush you get in a steep banked turn close to the ground. If you are not practiced in low level flying there is an almost irresistible effort to pull back on the stick which is why I think there is a prevalence of stall spin accidents with turnbacks. Practicing turnbacks at altitude is just not the same and again fosters a false confidence.
Secondly regular practice would obviously be required to maintain proficiency. For the average private flyer I think there are better things to practice then a manoever that covers a danger, the EFATO, that spans maybe 2 minutes of every flight. Pilots aren't bending metal on a regular basis because of EFATO's they are loosing control while landing, running off the end of the runway, etc etc. The less sexy but important fundamental skills are of more value practicing IMO.