some don't believe in that single paper u pointed out...some would get married but they cannot (maybe religious barriers, maybe the country where they r does not consider it legal, maybe a divorce to be finalized, so on so on); don't look at it from a perspective where u just need to want it, to do it, NO, maybe u want and u know that u find the one but in specific circumstances u CANNOT
also, some believe that a marriage could make their road to Canada more secure and fast (genuine or not genuine) and unfortunately CIC did question some very genuine marriages on the basis that they didn't spend much time together, they didn't live together, they married fast, etc...
on the other hand, if u get married u could apply right away, right? but with common-law u need to live together for at least one year...if any one would be a scammer which road will he take? wasting one year with some one he does not have any feelings or getting married?
yes common-law have to show more papers than the married ones but it's not too much more work....the most work and proof has to be put in the genuinity of the relationship and that they both have to show
as for the timings, that i really don't know; i really think there's a group of events affecting the process of an application and the region too; so it's really case by case scenario; but no matter the timings the important is the positive outcome
PS: don't rate me negative if u don't agree with me; (it happened before :-X)