In UK law jurors can be challenged on selection by both the judge and prosecution.
The arguments in favour of juries are:

A Jury best reflect the views of the society because of random selection from a wide rage of population. When a person's liberty is at stake it is a matter of principle that s/he should be tried by his/her peers.
The jury is regarded by the public as the ‘bulwark of individual liberties'.
Fact-finding is a matter of common sense and does nor require any specialized legal training
The opinion of the 12 jurors is better than the single opinion of the judge since it willbe more likely to prevent the individual biases.
It can be totally independent because it is unaccountable.
Juries are barometers of public feeling on the state of law, e.g. by deliberately acquitting against the weight of the evidence to express disapproval of a ‘bad' or ‘unpopular' aw.
There is no satisfactory alternative to a lay jury.
The presence of the lay jury ensures that the proceedings are kept simple.

Should a juror make it through the selection procedure, despite "shortcomings" It is one voice among 12. A majority of 10 is acceptable in most circumstances. It is the averaging out of views that make the jury so important. A "bad apple" would also require a very strong personality to slew the proceedings. Under UK law no analysis of jury decision making is allowed, so the impact of one "stray Juror" is a total unknown. .