I think that if there is one flaw in what Cutler writes (or at least how other people interpret what he writes) it's this...
He focuses too much on mercury and silver fillings. I think mercury is the most common heavy metal problem and fillings are the main source that a lot of people get their mercury from. However, mercury is not the only heavy metal people may have a problem with. Lead and other metals can also be a serious problem and can be gotten from other sources.
Just because someone removed their fillings years ago, it doesn't necessarily mean they haven't had more recent exposure to metals. First reducing the overall metals load with DMSA would be the prudent thing to do just in case you have other metals floating around in your system. This way you can reduce the risk of lead penetrating through the brain barrier along with the ALA.
I read info from Cutler again. You are right, he does say you can use ALA by itself. He just recommends its use in combination with DMSA to make it more effective.