For
- Increased head protection in falls and collisions (like with that low hanging branch you don't see until too late.) Having crashed in the past, this one is enough for me.
- Sun protection for those of us who are 'folicularly challenged'.
- Can be stylish.
- May be legally required as @freihiet mentioned.
- May increase visibility as @neilfein mentioned - I add 3M reflective tape to mine.
- In cold weather you can tape over the vents, and use the helmet to hold a skull-cap or balaclava on to increase warmth.
Against
- Even the ones with the best/most vents are hotter than no helmet.
- Some (not necessarily scientific) studies and anecdotal evidence shows that cars pass cyclists in cycling specific attire, including helmets, at a closer distance than they pass cyclists in street clothes.
- Some (not necessarily scientific) studies and anecdotal evidence shows that when protective gear is worn, the wearer tends to engage in riskier behavior. I personally ride faster and corner more aggressively, and I think I am more assertive about taking the lane.
- Protection in a collision is only marginal, especially if it is at a high speed or with an automobile.
Just a note about that last point - I hear that one a lot, and my usual response is that by a huge margin most accidients and collisions that result in injuries sugnificant enough to require medical attention (and therfore be tracked and reported) are not collisions or accidents involving automobiles. The most common is a 'fall' - meaning you hit something in the road or path, or otherwise lost control and fell over all on your own.