Crown instead of core in composite after root canal?

This is a question that I was just asked by questioner on allexperts.com. I thought it was a good one, so I am posting a longer version of the answer here as well. It is a question that many patients have after a root canal. Why can't I have a filling instead of a crown? I usually have a separate answer for anterior and posterior teeth.

For anterior teeth, after an endodontic procedure I often recommend just filling the endodonic access hole. These teeth often experience a considerable lateral force component and preparing them extensively for a crown can weaken their ability to withstand these forces. If of course, they are missing too much tooth structure I will crown them, but otherwise I will often fill them. If they need too due to darkening of the tooth, I will often recommend a porcelain veneer. This restoration requires less tooth preparation than a full crown.

For posterior teeth (Molars and premolars) I almost always recommend a protective crown or onlay restoration. Not only do these teeth experience a greater percentage of vertical vs horizontal force loading, but they also are under greater stress due to their position in the jaw. The further posterior a tooth is in the dental arch, the greater the occlusal force it experiences. In my opinion they will more likely fracture than anterior teeth without cuspal protection. Both crowns and onlays can offer far better protection than a simple bonded filling or core will.