When training for my IM last summer, I did an organized century. I had my aerobars on my bike. No, I didn't use them when in big groups and I only got down in them when it was safe. To be quite honest, tho, I'm a far more stable a rider in aero position. The center of gravity is so different, and I'm very safe and stable when in that position. I'm more wobbly when I'm upright (except when cornering- then I'll sit upright). It was a small ride, and most of it was a solo effort anyway, so it's not like there were scores of people all around to make it unsafe.
Use them if you need to. Don't use them when you're unsure of others around you. Roadies always scoff at me when I show up to rides on my tri bike. It's the only bike that's truly comfortable to ride (and climb) on, so I'm riding it whenever I want. I'm not stupid and I know when to be in aero position. Sorry-that was a minor rant from the ride I did on Saturday. Like I'd be so stupid as to endanger others.... I think those men were just mad because I was passing them going up a hill in aero postion while they were struggling upright. Oh, the ego sometimes...