I agree. Now, if you were putting her in a trailer that did not have a 5pt harness and the roll bar, among other things, then I would probably feel differently. But with trailers that are designed not to flip when the bike does and have saftey features if it actually does, I don't think you have as much to worry about.
My kids don't wear their helmets in the trailer. They're completely enclosed and we only ride on the bike trail where we possibly come into contact with one or two bikers/rollerbladers...that's it. They always wear them when riding alone and of course if I had them in a bike seat I would definitely put the helmets on. It was just too uncomfortable looking when they had them on in there. Common sense IMO.