If you don't trust the manufacturer who put the code into your handset, how can you trust that the security plug will always work (other than checking the internal wiring yourself)?
It is a complex trust situation. In this case, you can reduce the number of agents you have to trust by using the security plug. This is good for security.
The handset manufacturer could still be spying on you, but if the security plug actually works as advertised it would disable all attacks that would listen in on your mic. These attacks could be deliberate by any of the companies that have code in your handset, or it could be via an accidental weakness in any of this code that is exploited by a third party. This last kind of attack is what the linked article talks about, and a security plug would actually reduce the severity of such an attack.
If you trust the wiring, of course ;-]