The only time I see backdooring working well is when they have 3 or 4 top and you backdoor their mid as a team, THEN are able to hold them off and come back up and take top. If you are talking solo backdooring as someone like rammus, shaco (more effective than most, but still bad idea), zil (with tons of mobility troll build) or **** like that, then it's just costing your team. I also hate when people try to push 2 bot and can't pull it off while top is getting slayed. Don't try to double up on bot lane unless you are 80% or so sure you can cap it, otherwise it's another bad move I see people make.
I guess there is one other scenario I've seen where it can kind of work and that's when you have 3 people holding top who manage to kill 1 or 2 of them and you have time to at least cap their mid with your 4th to make it neutral. This can buy you some some extra time for top lane to back and buy.
Backdooring though is almost always a sign of fail players and team. There are very few times when it offers any sort of tactical advantage. I will say this, however. If they have 4 sitting top and you can't push their top tower, you might as well, AS A GROUP, try to take their mid and force them out of fortified positions. This is really the only "backdoor" strat I've seen used effectively, and then only as a last resort.