Allow me to put in a more strongly worded opinion about bicycle helmets. If you are going to get on the bike, wear a helmet! If you don't have a helmet, go get one before you ride another mile. They really can save your life when (not if, but when) you have an accident. As I was told at the bike store, "Remember that a helmet costs a lot less than a CAT scan." Indeed.
Maybe a story involving my own stupidity will help illustrate this point. No shit, there I was riding down Atlantic Avenue just as fast as I possibly could go. See, Atlantic is a big steep hill with a speed hump at the bottom of it. To me, that looks like a ramp at the bottom of a tall hill. Naturally, I decide to jump off it. I'd jumped it a hundred times before in excess of 30 mph so I had no reason to doubt I could do it this time too. Well, on this occassion the rear wheel hit first, followed by the front wheel. Unfortunately, the front wheel turned as it hit causing it to taco and me to hit the ground hard (since I was going so fast). Among other things, my head smacked into the ground pretty hard. The helmet sustained two large cracks in it but luckily, my head was undamaged. Were it not for that helmet, I could be dead today.
Now that was an instance of me being stupid but the helmet taking up the slack for me. There's plenty of instances where you could just be riding along minding your own business when a car pulls out in front of you, or you have equipment failure, or any of a number of things. Point is, accidents can happen at any time even if you're doing everything right.
Spending the $50 on a good helmet is a smart move and could save your life one day. Don't trust those idiots in traffic to have your well-being at heart. Take some precautions before getting out onto the street with them. Don't make me say I told you so.