In what circumstances is the riot shield useful?

Is the riot shield really only useful for team situations where you have one person defending while the other shoots over him? It doesn't seem the riot shield has a lot of benefit other than protecting your back when not equipped and protecting your front against an attacker while someone behind you shoots at them...

Is that the case, or are their other benefits?

The riot shield is really a situational "weapon." If you're trying to hold a location, pairing it with something like Blast Shield can give you a significant advantage. Backing yourself into a corner and crouching down can give your allies (or you) more time to capture a point.

As far as offensive capability, pair it with a gun that raises quickly (like a pistol) and Sleight of Hand Pro to allow you to quickly switch tactics to match those of your enemies. If they're firing at you, the shield will keep you covered, and if they're retreating, reloading, or trying to flank, you can switch to the pistol to take them out. Most thrown explosive equipment (semtex/frag/knife/tac grenades) can be thrown fairly quickly, especially if you're using Quickdraw Pro.

You move slower with the riot shield than most other weapons (Bora points out that without the Speed proficiency, it is in the speed class below LMGs and sniper rifles) so flanking isn't really the riot shield's forte. Also watch out for people who are smart enough to aim at your feet, or who are carrying semtex, as both of these can negate your defensive bonus. Melee'ing with the shield isn't as fast or as powerful as the knife, and coupling this with the movement speed hit means that you're usually going to have to back someone into a corner in order to get kills with it.

I've never found the "on your back" protection to be all that useful - if someone surprises you and attacks from behind, they're likely able to hit your legs or your side, so the benefit of the shield is relatively negligible.

As you've noted, you can draw fire with the shield, and give others time to deal with attackers. Air support and other killstreaks will also shoot at you, even though they can't hit you. You get bonuses for deflecting bullets (up to a certain cap per game) so you can make some quick points this way. Since air support and other killstreaks tend to only fire on one person at a time, you can also provide a distraction while teammates take them out.

In my own usage, I tend to see a good amount of people get frightened when they see a RS in a close quarters area. They have no way to flank and it throws them off a bit. I tend to use it on maps that have tight hallways or alleys, or anything close together. If they can't shoot you and can't flank you, whack them a couple times and see how mad they get when the game is over.