I think people are missing the point of herding the walkers. Here are the two most popular opinions and why neither worked for what the group wanted to accomplish.

1: SET THEM ON FIRE! Most people mentioned resources as a reason not to, another mentioned attraction of humans (i.e. Wolves) and both are part of the answer. The other part is that even if they burn all the walkers, the smoke will rise and attract more walkers. And once the fire dies out, they'll be left with a pit of burnt walkers and fresh walkers still streaming in.

2: BARRICADE THE PIT! While this is a good idea seeing the space they have up front, remember that the trucks in the back on the cliff are the ones the group worry about. This is because even if a truck falls (as it did) and there are more trucks or large cars behind it, the herd will build up and over a car (which is only 5' tall compared to a semi at 10'). Or if they can't or the group finds larger vehicles, the weight of the herd plus the extra tons of cars will slide off the cliff with a good rainfall and some erosion. Thus a free path for walkers to escape.

Anyways, what Rick decided to do was not safe in the end but it would have solved the problem of getting rid of a large noisy pit. They demonstrated that they could keep the herd together and curve them around in an intersection which is pretty impressive. The fault here lies in who rang the bell? I personally think it was Deanna's (now) oldest son but then again, that could be what they want us to believe.

The gripe I have about herding the walkers away: who chose the route? After what seams like an hour or two off herding, they're on a road that parallels the walls of Alexandria and seams to be only a few miles at most away! If I remember correctly, they said the pit was 3 miles to start with...

tl;dr: Setting the herd on fire would still draw too many walkers. Barricading the steep ramp in the back would only cause more erosion and more holes. I think Deanna's oldest son rang the bell. Who chose that route?