I think most of us are against showing guinea pigs because they're so easily scared and startled, and it's just too stressful for them. They're not like kittens and puppies and other small animals, that enjoy lots of attention and petting from all sorts of people. It takes them time to trust even the people who care for them everyday, and moving them to sites with noises and lights and lots of people and animals is just not something they enjoy.
I know 4-H promotes breeding animals, and I can see that for farm animals. But pregnancy and labor are very hard on sows and pups, and there are lots of genetic diseases. From what I know about it, which admittedly isn't much, 4-H doesn't put nearly enough emphasis on those issues when it comes to breeding guinea pigs. I think 4-H encourages it because they can be bred in urban situations, and don't require farms like cows/pigs/horses/etc do, but I think it's misguided. There's no way I'd put my pigs through that.