Bondo has gotten a bad rap over the years... since back in the days that high-school kids would just buy a bucket, slap it on the side of a car without even shaping metal, and then the inch thick piece would fall off. Duh. Like anything else, when used by a professional, it can give amazing results.
Bondo (or plastic body filler)
does have a place in modern body work, even on classic cars. The factory used it. The companies that make plastic (and nylon) body fillers have greatly improved the technology that goes into the mix. This isn't the bondo you used to hide the dent on your father's Buick anymore.
Like Mark said... shaping and stretching the metal to the best possible shape and then feathering in a bit of plastic filler to fix a surface... that's the easiest and usually best way to repair the flaw.
How many people do you know that really know how to use lead? Okay, so we all know "this one old guy..." It's expensive, it's poisonous to the user, and it's extremely slow. It's a difficult art form to master properly. If lead really was that superior, then major shops would use it. Insurance companies would insist on it for "proper" repairs. Auto-body shop schools would teach it as a preferred repair method. They don't.
People who advocate (and have lead on their cars) like to brag, "My car has no bondo at all... all metal." Well, that's not a perfect chassis. They just have a different type of body filler. You could use dog crap or peanut butter and still be able to brag the same thing.
I'm not suggesting anyone go slather Bondo all over their cars. Like paint, primer, welding, etc, etc... plastic body filler needs to be used properly on a correctly prepared and shaped surface. If you take the time to use the product right, then there's nothing wrong with it.
-Rusty