http://www.cyclehelmets.org/1139.html . I don't think rotational injury can just be waved off, as it is important to brain damage... just what degree helmets may contribute to this is not really known very well (also no real consensus, as far as I can tell). Again, I don't think it is so much that it is impossible to design a helmet that would be more effective (as noted, some designs are trying to address rotational injury, such as a sliding "scalp" layer), I just think bicycle helmets, particularly the most popular kind that just about everyone wears, are woefully inadequate if their purpose is to prevent brain injury. And I'm skeptical one could be designed that would still meet most riders' expectations of comfort and be significantly effective; I've worn a lot of safety equipment over the years in a lot of different activities (fencing masks, safety harnesses, motorcycle helmets, blah blah), and rarely is safety equipment designed for serious protection what I would call "comfortable".
What it really comes down to, for me, is that I feel I'm pretty unlikely to get in a crash, and if I am, it probably isn't going to be one where a helmet will help me much (getting hit by a car). Helmets really shine in lower speed solo crashes, and unless conditions are pretty nasty or if I'm riding in close contact with a lot of other riders in an event (or drunk), I just don't see it happening. And on the off-chance it does occur, I'm willing to deal with the consequences of not having a helmet, such as stitches for my torn scalp, in exchange for not having to put up with a helmet. We all make these trade-offs, and yours may be different than mine, either because you perceive a helmet to be more effective, or because your conditions are different than mine. I totally don't care if you wear one... I just want people to realize what limitations the equipment may have, because overestimating safety equipment is never good for anybody. If the result is still donning a lid, I'm OK with that. I don't think it will be likely to harm anything... I just don't think it's any more necessary than wearing one for walking for a lot of people.
There have been plenty of studies cited here that show helmets don't have significant effects on reduction of serious head injuries, particularly when crunching real-world numbers (in contrast to evidence presented that they do; conflicting evidence).