The definition of common-law is fact based, so you are common-law after living 12-months together in a conjugal relationship whether or not you declare it or sign a form together. You can't choose to become common-law, you simply are after 12 months of living together. This is the reason the declaration form is not a requirement for a simple spouse sponsorship. The VO is supposed to come to the conclusion a couple is common-law based on the facts, and simply stating you are common-law to a notary is not factual i.e. the notary will not be checking all your common-law proofs... notary is just confirming 2 people have told him they are a common-law couple. This is easy to do even if the relationship is fraudulent, hence the reason it's not a required document.
Now for when there is a co-sponsor/co-signer to the application, the declaration form is required. This is because CIC wants as simple proof of the common-law relationship with co-signer as possible... they don't want people to submit full proofs of a co-signer as they do with the applicant. So the declaration form is an easy straightforward way to do this.
As the declaration is not required by CIC for simple spouse cases, i really think the VOs that ask for it are simply confused at the rules or flat out wrong to ask. They should be judging common-law status on all the proofs in the application. However instead of arguing with a VO, of course it's best to just send it if they end up asking for it. Although the cases this happens is relatively small i think (would be curious to see if perhaps a specific visa office was responsible for lots of these requests)... so if you were not planning on going to a notary anyways I would still advise to not waste the time and money it takes to submit a form that CIC doesn't require to begin with!
