There are certainly different approaches that can be taken. I think that the idea behind the standard here is that it's only in the clearest of cases that you would want, essentially, the executive to prevent Parliament from being able to have a debate on whether it wishes to go forward with a certain piece of legislation representing a certain policy solution to a problem.
The report that a Minister of Justice might make is not the only source of information that's available to parliamentarians when they're considering legislation as to the questions of constitutionality. Certainly, there is a law clerk for the House; there's a law clerk for the Senate. In addition to that, at committee hearings there's often a wide variety of witnesses who appear to offer their views with respect to the legal issues, including constitutional issues on bills.
I believe that the purpose of the standard that's been adopted is to recognize the important roles that each of the branches plays and not to foreclose democratic discussion.