There are friction surfaces not protected by the rings. The rollers rub the bushings, and also the face of the sprocket teeth. To a certain extent, but not effecting length, bushing plates rub on pin plates.
Also chains rub on the guides. Most guides are made of high molecular weight plastic, but they will wear and even the mfg's of such materials will tell you that lube helps to eliviate wear even on material touted as non-lube.
O-rings do in fact hold in lubricant around the pin inside the bushing. More than likely it's oil instead of grease. Even filled with grease, it tends to soften to an oil state quickly.
What you use to clean your ring chain is as important as anything. A harsh chemical will tend to either dry out or swell an o-ring, neither of which is a good thing. Some lubricants are harsh as well, but with the integrity of the rings that mfg's use these days, just about any lube is safe for o-ring chains. Just as safe as the WD-40. Yes, solvents can and will harm o-rings, but not unless they are subjected to the chemical for long periods of time. Usually most solvent lubes will have the carrier evaporated before they can damage a ring. Plus if there is a sufficient oil left, it will help to keep the rings fortified. The worst thing you can do is clean your chain with a harsh chemical and let it sit. Once a ring drys out it will seek replentishment from wherever it can get it like a sponge.
If you aren't giving them adequate lubricant, they will get it from inside the bushing/pin area behind the seal.
Contrary to popular belief and even published in some credible articles, yet incorrect, is the myth that chains don't stretch they just wear. They DO and WILL stretch. This is usually seen at the very first rides. This is due to the plates actually elongating. It's very minimal change, but a change indeed. That is why some of your better chains today are "proof stretched" at manufacturing to eliviate early adjustments due to stretch.
Any legth change after the intial stretch period can be attributed to friction surface wear.
And, sprockets will not, and do not wear to a noticible change without chain pitch growth. There may be a bit of change in the seat of the sprocket between teeth from rubbing the rollers, but any hooking or pointing of the teeth will definately be from chain wear showing itself on the sprocket. (This can't be said true for big monster bikes with soft aluminum sprockets)
IMHO, WD-40 is a bad choice for a penetrating water dispersant. It only leaves a little silicone behind to serve as a lubricant. Now, as some have mentioned before, there are lubes fortified with teflon that you can spray on and they will not only serve as a great water dispersant but you will be left with the benifit of a good lubricant, although the lube tends not to stay too well.
The very best lube you can use on any roller chain is a penetrating molebdenum disalfide based fluid.
Dry-film moly, as it's called, is very thin and will penetrate and clean all of the reachable friction surfaces while leaving a treatment of lubricating solids. Lubricating solids are far superior to film carrying fluids such as petroleum oil. Not only will moly carry a superior load, but can handle shock load far better than petroleum films. Offroad specific brands of moly dry-film will contain anti-sling additives as well to help keep the lubricant in place even at high rpm's. The most noticible thing about dry-films are how clean they are. They usually have the consistancy of water and can be wiped off with a rag leaving your chain looking like new, while providing unsurpassed protection inside as well as cleaning and fortifying the o-rings. End result is a happy chain that gives you long life, and is as easy as rinsing off with a hose to clean. Dry-films were designed originally for an industrial setting where the attraction fo dirt and grit can't be tolerated. They fit our sport perfectly.