It's a question of what the implications are of making that choice.
We could say, okay, your social security number is now your unique identifier, so let's start putting your unique identifier against every piece of information that the government collects about you, maybe at the federal level, but maybe also at the provincial level and the local level. Someone could come along and say, well, let me see if I could query a whole range of databases and start pulling together information that I know is specific to you and then use that to create a profile of you that may be helpful, but I may chose to do things with that information that are not helpful.