6 or more in the circuit is usually a function of a busy GA field and many airfields do limit the amount of circuit only traffic.
In my limited PPL experience, flying a ciruit is usually a precursor to making a landing and this is what most of the 6 (or more) will be looking to do. ;-)
Anyway, if the airfield is A/G or AFIS, how does a "maximum number of aircraft" policy get policed?
IMHO, it is not the shape of the circuit that is the blocker at busy GA fields, but the inability of many PPLs to fly a nice tight circuit.
The US PPL training, in this respect, tends to produce better results in my experience.
Personally, I'm a fan of the oval circuit, but would hesitate to use it in an environment where it was not standard since, like the RIAB, the dangers are not inherent in the manouevre, but rather in the gap in situational awareness that they may create for some other pilots.