Backdooring is a strategy. Just like any strategy, it has its time and place. I've actually lost games because no one *would* backdoor. My team would just let ourselves get bottled up trying to cap our bot and mid leaving their three points unthreatened. Should you play only backdoor? No. It'd be like if everyone played bot. You need to be flexible. A strong champ like Maokai can backdoor and hold an enemy bot or mid against two or even three enemy champs, giving your team the chance to break the bottleneck and take more points. A champ like Teemo or Twitch can stealth and sneak out of your base even if the enemies have the Fog of War exits covered and still give your team a chance. An assassin like Talon can backdoor and if they send only one champ to counter him, he can probably kill them and continue to neutralize or cap.
Someone doing nothing *but* backdooring is still contributing, if they do it well. Neutralizing or even capping a point gives takes the pressure off. HOWEVER, backdooring can backfire. If you can't even neutralize the enemy point without dying, then backdooring did nothing to help your team. Your skills would probably be better used elsewhere.
If you're backdooring, you have to judge whether to stick with it or run away when the enemy comes. If you're alone and they send three champs to interrupt your cap, you run. Run like a little girl. Because if they've sent three champs, then YOU'VE ALREADY DONE YOUR JOB. And if they send one champ, and you can kill that champ and keep capping, then you've done your job (assassins can make good backdoors for just that reason).
If you see someone backdooring, don't just yell at him... because if they counter him, your numbers will be better in your battles elsewhere on the map. And if they don't counter him, you've got a free point. Keep pressuring in your battles. If that Teemo keeps the point he just took, they *have* to move to counter, which means you can win your battle, and you can pull ahead. You need to attack defensively... don't commit, poke until Teemo has capped that point. Then act according to the enemy reaction. If a player or two leaves to take back their point, then you commit and take back yours.
People rage because useless backdooring can cost the game. Well, so can a useless bot... or any useless player. It's the dom equivalent of a support. A good backdoorer can win you the game. A good one won't carry the team because like any support, your teammates have to be able to take advantage of it. A bad backdoorer is just that... bad.
* Rammus or Maokai: With BoM he can get there fast, and with his naturally high tankiness he has a good chance of defending a captured point. If he can't take it, he has a good chance of escaping. In this light, he's acting like a tank *should* act: taking the focus fire so his team can work elsewhere.
* Talon or Leblanc: If that enemy mid is just sitting there, you can start to take it. If they only send one champ, you have a good chance of killing the champ before they can do anything (i.e. Cutthroat > Rake > Noxian Diplomacy) and even do it on their turret while only having to tank one turret shot.
* Teemo/Twitch/Eve: When your team gets bottlenecked, and you're all stuck at base because the enemy has capped your two points and you can't break out, a stealther can make it past the enemy and backdoor. That alleviates the pressure at your camp and can let you break out and survive.
Good times to back door:
* Your team doesn't need you and that point is empty.
* Your team is bottlenecked and needs to break out. Keep in mind that if you are bottlenecked, they have the Fog of War covered. A stealther is your best bet for a backdoorer. A better counter, however, might be to send all five champs against the weakest point, take it fast, then overwhelm their reinforcements. This depends on how fed they are.
* You are a tank, or are acting like one, and pulling focus fire to lessen the pressure on your teammates at other locations. PULLING FOCUS FIRE IS USELESS IF YOU DIE IN THREE SECONDS. You need to be able to withstand a few hits. That means a tanky build.
* The other team has fed too much and you can't win in fights. So run around taking empty points and run away fast if they come to counter. THIS IS A VERY RISKY STRATEGY AND IF YOU DON'T PULL IT OFF YOU CAN ENSURE YOUR TEAM'S CERTAIN DEFEAT.
NOTE: I don't backdoor often. Normally I play bot. These are examples of games I have actually played where backdooring has worked both with me as a player and others. They are not the be-all and end-all of strategy. If anyone has counters to these strategic uses of backdooring, please by all means mention them, because more knowledge = better strategy.
If all you can say is backdooring can cost you the game with a useless player, well, I've already said that. Try to contribute something constructive.