The sharper the point, the more likely it will penetrate clothing, skin, bone, muscle. All that is needed for dusting a vampire is wood to enter the heart. A pencil would work too.... but unless you have strong magics behind the pencil, it probably won't make it thru the rib cage. If it hits bone, it will break.
The sharpest wood point will dull at least somewhat when it impacts bone. While bone also will dull metal to a degree, the harder the metal the less effect it will have. Most modern arrow heads are actually little bits of engineering -- they aren't just pointy and razor sharp, they actually well balanced for flight and they are designed to cut on impact in way wood simply cannot. If you aren't going to get a second chance, you want to be sure your arrow goes in.
Another thing to consider? Buffy wasn't just hunting vampires. While wood kills vampires, some demons are likely scaly or have harder hides that require more penetrative force than wood could give. Some probably are killed by iron or other metals.