I went to dental assisting school, worked in an office under the direction of a dentist who now teaches other dentists, and I still work in the industry, so what I say comes from a place of actual knowledge of the industry.
When you have a root canal, you're removing the vitality of the tooth. It's dead and it no longer has blood flow. Over time, the tooth will become brittle and is very likely to break. There is no guarantee on time. It could happen in months or years. Your genetics and habits will make a difference in the longevity of the tooth. If it's a front tooth, you'd probably have more time with it. If it's on the side or near the back, it's exposed to considerable force constantly. Some dentists will let you "get away" with only having a filling done to close up the access area on a front tooth, but that's only valid if it's a front tooth and it needed the root canal due to trauma or a "random" abscess. Most of the time, the tooth also has decay, and that's what necessitated the root canal in the first place. If your decay was so severe that you needed a root canal, a considerable amount of the tooth is already compromised, meaning it's going to be very weak without full coverage.
Look at it this way. Root canals, at least here, generally run between $600 and $900. If you've invested that much in saving a tooth, you don't want to risk breaking it after. Hence, the crown. And, you do want to save your teeth. If you lose one, you're going to have more difficulty eating, your other teeth will shift from side to side, the opposing tooth will literally start growing up or down in the direction of the space because there is no longer opposing force on it- these things can cause considerable pain because your bite is no longer in alignment. Plus, when you lose a tooth, you lose the bone around it. It starts getting absorbed by the body. Your jawbone melts away. That's why the mouths of elderly people pucker in. It's not age- it's facial collapse- bone loss. So, it's a million times better to save the tooth with a root canal and to protect it with a crown after.
Any and all of my posts are not intended to diagnose or treat any type of medical or dental condition and should be used for entertainment purposes only. Should you have a question relating to a medical or dental need, please seek out the advice of a qualified professional, not some chick online, no matter how brilliant her answers and credentials may seem. :p
I agree with JA. I let a molar go too long after a root canal and the tooth split down the center which made it uncappable. I had to have it pulled and did an implant which cost about 4 times as much as a cap. Do the cap.