Wow.. some crazy answers here.
Consider this, sport scientists have taken various athletes and measured their lbs of force for a punch, kick, etc.
The ones that top the list are generally professional fighters (pro boxers, pro MMA fighters, Muay Thai etc.) and generally have never broken a board in their life.
Board breaking is a good measure of commitment to a strike, and it is a confidence builder. But it doesn't make you hit any harder. Good mechanics and repetetion does that.
Technique and power results you gain far more from hitting a heavy bag then you do any board. One because the bag has some give to absorb your power. Two because you slowly train your brain to allow you to hit harder.
You see a major hurdle in striking hard is your brain's automatic reflex to prevent you from injuring yourself. Unbeknownst to you, your brain will only let you hit so hard, it is going to pull power off. The more you hit objects, the more you condition the brain to put more and more power behind it because it realizes that X amount of force isn't going to injure you.
Hitting hard objects takes WAY longer to develop this, because by nature you aren't going to hit a hard object with enough force training wise to build this right away, or you will hurt yourself and put yourself further back.. for example, you can't hit a brickwall as hard as you can a heavy bag.. your brain will not allow you. Furthermore you can hit the heavybag repeatedly, the brick wall you are only going to get a good hit in, cause yourself some pain and possible injury. As a result, your brain is going to automatically take even more power off the next time you try to hit that hard object, since it learned that last time hitting with X amount of force caused pain.
It's the reason they start with easy breaks. Because it is a confidence builder. Give me a room full of people and in 20 minutes I can have every man, woman, and child able to break a board. It's a popular motivational speaker trick.
Striking hard objects that such as bricks doesn't condition your striking areas, at least not practically. You gain way more by hitting something that has some resistance but still has give and allows you to hit it as hard as you can without injury.
Some of the hardest striking styles do not ever hit hard objects other than people. Kyokushin Karate spends a whole lot of time punching other people, their power is in mechanics and practical ability from sparring.
Muay Thai fighters use heavy bags.. that whole banana tree bit is from when heavy bags weren't around.. banana trees by the way are soft. That is why they are used, not because they are hardcore and going around kicking oak trees. It is because banana trees have soft bark and lots of give. A lot like a heavy bag.
Boxers hit heavy bags, pads, and people's head.
I think breaking serves a purpose for confidence building with kids, and maybe some adults. Personally I think any time spent attempting to break is much better spent sparring. You will gain far more confidence as well as practical ability in actually utilizing techniques, developing timing, and learning to get a feel for openings in defenses, and your own openings.
The confidence from knocking the wind out of an opponent, or sending him flying with a kick is far greater then the confidence gained from breaking a piece of wood. The lessons learned with it are also far greater in practical skill.
Unless of course you find yourself being assailed by boards, bricks, and tiles regularly.
Just my opinion.