So what? I say bring 'em on.

I find noisy planes as irritating as the next guy but wouldn't it be great to see a sportsman class with 6 guys flying small gas powered IMAC style
planes? Better than three guys flying pure pattern planes. Ready made pattern pilots don't appear out of thin air, fully committed to the "purity" of pattern. They have to start somewhere and often that is with an affordable, quick build small gas plane. These guys are some low hanging fruit for pattern recruits. Alienating them is shooting ourselves in the foot. Get them hooked and worry about getting them pure later.

Most new pattern pilots will try their first contest at their local field or a nearby club. They will fly whatever they have. If you don't get them here, you won't get them anywhere.

You are not going to save any fields by prohibiting a noisy plane from flying in a pattern contest if the same guy can fly there the other 363 days of the year. If the field has a noise problem then fine, no fly.

Pattern purity = country club mentality = declining participation.