The illusion of justice is more powerful
Lie-detector tests increase the public faith in the justice system, which creates a more stable, law-abiding society.
If the accused believes that they will be put through a lie detector test which can detect lies to a high level of accuracy, they are more likely to decide to come clean and tell the truth. 
Lie detectors are already used by the UK’s probation service. The Ministry of Justice found that sex offenders who took a polygraph test were twice as likely to admit to officers if they broke the terms of their release. Seven police departments in England and Wales currently use lie detector tests to monitor high-risk individuals on probation. Therefore, polygraph tests should be used in criminal courts, not as a tool for convicting suspects based on their lies but to induce a confession. The results do not even need to be recorded. There are effective ways to use a polygraph test to induce a confession without ever looking at a single metric gathered. In the US, prior to 1988, employers could use polygraph tests to screen candidates for jobs. It was commonplace to perform an “acquaintance test” to demonstrate to the suspect how “effective” the lie detector test was. The examiner would ask the candidate to pick a card from a deck of cards and instruct them to give the answer ‘no’ to every question the examiner asked. The examiner would then ask if it was a red card if it was a picture card, a club etc. The examiner would then “deduce” which card the candidate was holding based on the lies they told. In reality, many examiners admitted that they had manipulated the deck and always knew the card the candidate would select going into the test. But the belief that the machine was accurate was often more powerful in inducing truthful responses than the actual results of the test. 
In instances where lie detector tests have been used to induce a suspect into making a confession, there have been a high number of false confessions. In Chicago in 2013, Chicago police began unofficially and illegally using polygraph tests to induce confessions from suspects. Only a small percentage of the test results were even recorded, as the results were not important. Instead, the police officers told the suspects that they had failed a polygraph test, even when they didn’t, in an attempt to obtain a full confession. When the suspects were repeatedly told they had failed the lie detector tests, some offered false confessions in the hope that the interrogation would end. ISome of these suspects ended up spending years in prison before DNA evidence later exonerated them and secured their release.  Younger suspects, those taking the test in their second language and those with mental disabilities were most at risk of providing a false confession when confronted with a lie detector test. However, anybody, when subjected to more than 50 hours of aggressive interrogation techniques without sleep, may find themselves sufficiently pressured into giving a false confession. Using lie detectors to obtain a confession effectively amounts to getting the victim to provide a confession under duress, and should not be admissible in court.
[P1] Lie detectors help secure confessions. [P2] If they were a mandatory part of criminal proceedings, more guilty suspects would confess. [P3] Therefore, they should be admissible in court and used in legal proceedings.
[Rejecting P2] These confessions are made under duress. This means a higher percentage are false confessions. Therefore, confessions made under the threat of, or during the use of, a lie detector, should not be admissible in court.