There is an argument for the completely professional judiciary. In many civil law systems, should you come before a court or tribunal, the person presiding over the case will be a fully trained and qualified judge. In both England and the USA non-legally qualified people continue to play a key role.
The right to trial by jury is entrenched in the VIth [Criminal cases] and VIIth [Civil cases] amendments to the US Constitution. In England (as entrenchment is incompatible with the doctrine of the sovereignty of Parliament), it lacks that protection - and a number of attempts have been made to reduce the availability of jury trials.
The argument for jury trials can be summarised as follows - Jury trial "is a fundamental right and an essential safeguard for the rule of law. Juries provide a barrier against oppressive and politically motivated prosecutions. They enhance the transparency of the court process and promote public confidence in the criminal justice system." (per - Law Society)
A more detailed argument can be found in the following article
In England lay magistrates continue to play a key role in the legal system. They deal with about 91% of criminal cases [95% of cases are finally dealt with in Magistrates Courts, but the 4% difference reflects the role played by Stipendiary [legally qualified Magistrates who sit alone, usually in the larger urban areas] Magistrates] - only the most serious are heard by a Judge and jury in the Crown Court. A detailed discussion of the value of magistrates can be read here.
Direct Gov has an article about becoming a magistrate and the work it involves.
