Carpet protection is extremely important for your carpet, but when talking with most people, I find out that they don't know exactly what it is or what it does. In my years of cleaning, I've met people with almost brand new carpet ask to have carpet protection applied, and I've also met people who need their carpets cleaned every six months (*raises his hand*) who don't think they have a need for carpet protection. Both cases were wrong.
Carpet protection has many names, most of which are brands that you may or may not have heard of. 3M's Scotchgard, DuPont's Teflon, or whatever other name used, normally have the word "protectant" in the name by other manufacturers. They all do pretty much the same thing, repel water and resist soiling. Protectant has some great benefits when used as it is designed.
Let's begin with what it does. In simplistic terms, it creates a short-term water resistance, giving you more time to clean up something that has spilled on the carpet. It also has a long-term effect. It helps to keep daily dirt from settling as hard into your carpet and makes the dirt easier to remove at your next steam cleaning. Protectant isn't a cure-all, but it does give you more time to clean up spots. Normal traffic dirt also vacuums up better.
Now, let me be clear about what carpet protection doesn't do. It doesn't stop stains from umm... staining. Blue Koolaid, for example, will dye your carpet blue. Yellow mustard dyes your carpet yellow, and a green permanent marker will... well, you get the idea. A general rule of thumb is that things that have color are normally from a dye that will actually change the color of your carpet, not just get it dirty. Protectant doesn't keep things from soaking into the carpet if left alone or keep dirt from getting ground in if it's not cleaned out in a timely manner.
Almost all new carpet has some type of carpet protection on it. That's where the idea of a stain warranty comes into play for new carpets. These warranties normally concern how long the carpet protection will last, not that your carpet won't become stained during that time.
Enough said. Here is my personal recommendation based on both manufactures recommendations and more importantly, on field experience in real-world applications. You don't need carpet protection for new carpet during the first 4-5 years. It already has some type of carpet protection. From 4-5 years on, it's good to have carpet protection applied every 2 years or so. It's not that the carpet protection stops working at some point, but just like a Teflon pan. it will wear off with use. Look at an old, used Teflon pan -- it will, at some point, start to wear thin, and scratches will start to show through the metal. The same thing happens with your carpet. The less you walk on it, the longer the protection will last. The more traffic it has, the more often you'll need to reapply.
Why every 2 years when new carpet can last up to 5 years or beyond? Think of it as a booster, not the full treatment. The carpet manufacturer is capable of treating the fibers specifically, but carpet cleaning specialists can only topically treat the whole carpet.