I would start with the premise, instead, that before creating an exception to the principle that everyone is subject to the rule of law--this is a remarkable exception--Parliament ought to have compelling evidence of a need by law enforcement to resort to such extreme conduct as committing assaults, engaging in threatening conduct, forcibly confining individuals, or engaging in firearms offences that could pose, although not an intention to cause bodily harm, real risk to individuals.
An additional harm is to the very concept of the rule of law, in that the state ought to be setting an example to citizens. When the state engages in unlawful conduct or conduct that we as a society view as morally reprehensible, it undermines in a fundamental way the respect of society for those norms.