I'm no expert, but there are a few fairly obvious points to start:
Planks are more easily destroyed by creeper explosions than (for example) cobblestone.
Planks can catch fire and are consumed, which isn't true of some other materials.
Cobblestone is readily available in practically unlimited quantities, whereas wood for planks is less common and requires a little crafting--so it's a less efficient use of your time.
One advantage is that you'll probably get sick of seeing nothing but stone buildings after a short while, so a plank structure will look nicer in comparison.