First, I have a composite (Carbon Fiber) hood. This means two things: 1) The entire hood is a much lighter weight than a stock hood. 2) The material used (carbon fiber) is MUCH more brittle, and although carbon fiber is stronger than steel in some ways, it is much weaker than it in others.
Then you say, why hood pins? Well, this can be broken down into 2 parts as well.
1) Because the entire hood is lighter, there is more stress on the latch mechanism. Why, you ask? Well, a heavier hood exerts more downward force on your latch mechanism than the lightweight composite hoods do. When you are driving, airflow that gets under your hood from the front will exert upward force on your entire hood (because our hoods, like most, have the hinges in the back and latch in the front) which therefore exerts upward force on your latch. This force from the airflow (which is higher at higher speeds) is counteracted by the strength of the latch mechanism and the overall weight of the hood. With a composite hood, there is less weight and therefore more of that upward force has to be counteracted by your latch mechanism. There are a few different kinds of hood latch mechanisms on different types of cars. On OUR cars (E46's) the latch is MOUNTED to the hood, with screws/bolts, and as you bring your hood down, the latch clicks into the metal bar on the body of the car. On some other cars, this is reversed, and the latch is on the body of the car with the metal bar/ring being on the hood itself. On our cars, that latch on the hood itself is the failure point that hood pins will act as backup to. Why does the latch need to have backup you might ask?
2) Because the hood is a composite material (carbon fiber in this case), you cannot just weld the latch onto it obviously. It has to be bolted onto the carbon fiber itself. Carbon fiber is a brittle material. It cracks and bends very easily unlike metal. Although it has enormous tinsel strength, lateral forces will bend and eventually break it much quicker than steel. The airflow force on the latch will stress the carbon fiber around the mounting points on the latch. Another factor here that does not help is the thickness of the material.
All factors considered, Carbon fiber is strong. Under low stress conditions, the latch has a high probability of not failing. This means slower speeds and smooth roads. BUT we all know that we like to go fast, and roads are not always smooth. IF you hit a bump, pothole, etc., the latch COULD be damaged. Obviously this is not normal, and usually doesn't happen, but even if it did, you most likely wouldn't notice stress cracks on the underside of your hood around the latch right away, now would you? Because of CF's brittle nature, this scenario is much more plausible than with a stock hood. If your latch was damaged, or if you were driving into let's say a 30mph headwind going 95mph (definitely also possibly), your latch could fail. If that happens and you don't have hood pins, that hood is flying up. It will either completely blind you or fly off and possibly blind someone else, hit something, make someone swerve, etc. Regardless, it will scare the **** out of you as well, and possibly cause you to crash just from that alone.
In conclusion, hood pins are anchored to your car's frame/body and act as secondary latches. They prevent your hood from flying up during a latch failure. That is why I think they are necessary.
STORY: I had a Lexus IS300 over the summer with a recently bought CF hood. The hood CAME with pins, which I was happy about. TWO DAYS after I bought the hood and put it on (and 3 hours of fiddling with it to get the fitment right), a lady in a forerunner decided she really needed her coffee and took a left into a dunkin donuts from the other side of the road without checking to see if anyone (me) was driving in the opposite direction. I slammed my brakes, but it was too late and I hit her going 35 and this is what happened to the car:
The hood pins held the hood down even after the impact destroyed the latch assembly. That hood would have flown off if there weren't any pins on there. I used the money I got from the accident to buy the M3 I have now, and my first move was BUY AND INSTALL HOODPINS. Hope this helps you decide whether or not you need hood pins!