Simply put, partisan appointment of judges erodes confidence in the integrity of the administration of justice.
A fundamental principle of our justice system is that “justice should not only be done, but should manifestly and undoubtedly be seen to be done.” This stressing of the image of justice is informed by the overriding public interest in maintaining confidence in the integrity of the administration of justice. Even if you ignore the issue of whether the judges appointed would have an actual bias to convict and jail, the important issue of perception remains. Will a reasonable person perceive that the appointed judge is biased? Further, even if they are informed that the relevant judge has a duty to make fair and impartial decisions, I have doubts that the reasonable person would be assuaged of their suspicions of bias.
To take an extreme case, the executive branch of the government exclusively appoints the judges that may preside over national security offence cases in Hong Kong. In the first three years of implementing the national security offences, the conviction rate of national security offence cases was 100%. In that situation, can an assurance that the judges are impartial truly convince one that there is no bias at play? The majority of those charged pleading guilty may suggest otherwise. After all, assurances can only do so much when the deck seems stacked against you.
Granted, the legal system in Canada is not that extreme. However, for many, it would be unsurprising to learn that they feel as if that the justice system is already stacked against them by the time they get to trial. This may be for reasons such as selective policing and enforcement, or a lack of access to legal representation. If that is their experience, I wonder if their perception of bias by the system against them may in turn cause them to perceive the partisan appointed judge to be just another manifestation of this bias. Even if this is determined to not be a perception that a reasonable person would have, a partisan appointed judge certainly does no favours in creating confidence in the administration of justice when it seemingly creates another hurdle for an accused to defeat.