My exposure to 30mm flats has been limited; I much prefer the larger scales. While there are diamonds in both, what I've seen so far is that 30mm generally seem to have less overall detail and less consistent detail than the larger scales - especially in faces. I've also noticed that the larger scales, given equal painting skills and painted in oils, tend to look better to the eye; especially to the uninitiated. Acrylics seem to have a more even playing field, but you are still painting a 30mm piece of variable detail.
All that said, I am curious as to the motivation/reasoning behind the suggestion to separate the 30mm out from the larger scales. Did someone walk away unhappy...? Another question, and alfsboy hinted at this, is there any data on the average age of the 30mm flat painter? My guess is that they would tend to be older and require magnification. I know I do... Painting the smaller scales could be increasingly problematic and forcing painters to go to larger sizes. Ergo, fewer 30mm pieces on the comp tables.
Now, whether this might be grounds for separating 30mm (and perhaps up to 45mm) from the larger scales depends, I think, on the judging system. An IPMS first, second, third style carves everything up into multiple categories divided by scale/size, era, subject, etc. and basically spreads things out. It shares the wealth among 40-50 categories. The open, variable number of gold, silver, bronze medal, system uses two basic categories - out of the box 'painters' and 'open' for heavily converted and scratch-built subjects. It tends to reward the painter based on a measured set of skills regardless of size, paint medium, era, etc. Scale is not a factor. Since there's no limit to how many medals can be awarded, one big category should be ok.
Glen