It's hard to imagine a helmet having a negative effect during any kind of accident. It's possible, like you get a strap caught on a fence post and it snaps your neck in the fall, but not even the minimal extra weight is going to have any effect during an accident in terms of your head hitting the ground at any significant velocity or not. Sure, those extra couple hundred grams on your head may be just the difference to make your head hit the ground, but not with any velocity. If it were a question of whether your head would hit the ground with the helmet or not hit the ground without it, then even if it did hit the ground I bet you wouldn't even put a little mark on the helmet, that's how slow it would be going.

In terms of the helmet giving a rider an over-confident sense of security -- sure, it happens, just like when you have 4 wheel-drive and it snows and you don't slow down, or you think ou can take the corner in the rain because you just put new tires on your car. But you can't blame the tires -- or the 4 wheel-drive -- for any injuries sustained in the wreck, nor the helmet for your carelessness.