It actually puts a legal requirement to do the opposite - to hire the best candidate regardless of their race, national origin or their interest in yiffing at FurrCon.
At least in the US, (and I strongly suspect in Canada as well) it's up to a jury to decide if the given rationale is genuine or an excuse to cover bigotry. Anyone who knows anything about how to do the job is likely to get blocked at jury-selection. Imagine explaining to 12 literature majors why cubic-time lookup is bad, while opposing council wants to know how often you use key-value stores in your day-to-day work.
Not always. For example if you somehow learn that a candidate plans to take several months parental leave in the near future, they can't possibly be the best candidate, but you're forbidden from taking that into account.
If you believe for instance that
1) it takes 3 months of ramp-up time to do an effective job 2) after 3 months off the job, you need to ramp up again 3) the employee will leave in two months 4) the employee will be gone for twelve months
You could conclude that the next 17 months of this employee's tenure at the company will be ineffective.
You could contrast this with an employee that works straight through and will give you 14 effective months.
You might get an exceptional candidate taking the leave, but they have to make a huge impact in a short time to be better for the company on balance than an extra year of labor from a roughly equivalent candidate.