When aircraft first flew they could turn on a sixpence so the rectangular circuit was borne. The oval circuit followed primarily because forward vision was obscured nescessitating a curved approach. As the performance of military aircraft increased, an oval circuit was the only realistic option.
The average PPL student and many PPL holders require thinking time, an oval circuit correctly flown simply overloads them and does not provide them with the necessary time. A rectangular circuit therefore provides more time and a better lookout for the inexperienced.
That said, lets keep them a sensible size, Runway on the wingtip downwind, and not more than 45 degrees past the threshold for a Normal circuit.