I should probably add then, is the rate "from the promo" at the highest tier (e.g. 100% in some promos), or the lowest (because if you had gone to purchase that extra number of points, it probably wouldn't make it to a bonus tier)?
In the worst, latter case, it probably wouldn't be great value, especially if you sustain yourself in Aeroplan by purchasing points mainly, unless you were in a real hurry.
In the best, former case, when there is a promo on, if you can spot a few more dollars, this saves you a trip to the Buy Points portal, while you still get great value and without affecting your annual limit.