First of all, kev100 is not quite correct.
The current legislation stipulates that a person is not eligible for jury service if they have ever served a sentence of 5 years or more, or have served any period of imprisonment within the last 10 years. Also not eligible in England and Wales are those who have served any kind of community punishment within the last ten years.
I believe there is sound reasoning behind this. Persons who have fallen foul of the law are, in general (though, I accept, not necessarily), more likely than those who have not to have a jaundiced view of the judicial process. They are less likely to take an objective view and reach a verdict based solely on the evidence placed before them.
Similarly, I believe police officers and some others involved in law enforcement may well be less objective in their decision making, albeit for completely different reasons. I believe it was a mistake to no longer provide automatic exemption from jury service for people in those occupations.
I am by no means suggesting that all convicted criminals will be unable to reach an objective decision or that all police officers would convict anybody before them. However, the principle aim of the jury system is to provide, as far as is reasonably possible, a selection of people who will come to the Court �without baggage� and arrive at an objective decision based solely on the evidence. To allow into the pool a group of people who are statistically far less likely to be able to do this would be irresponsible.
Circumstances often arise where the rights of the minority must be overridden by those of the majority. Unfortunately this is often overlooked in the current climate. The public has a right to expect justice to be fairly administered. If this means that the rights of a few convicted criminals are subjugated by them being barred from jury service, so be it.