Burton is talking about Eastern Canada, where it is in fact disgustingly hot in the summer and it doesn't cool down appreciably at night. I have not-so-fond memories of my undergrad days in Southern Ontario trying to sleep at night without A/C, where the only way to get comfortable was to lie naked under a wet sarong with a fan pointed on it for convective cooling action.

In the Rockies things are more reasonable, with nighttime temperatures typically in the 4-8 degree (C) range. I usually use a -3 down bag in the summertime and a -12 in the shoulder season. Sleeping bag ratings are "survival" ratings, and everyone has their individual "comfort" ratings. I usually add 10 degrees (C), or maybe 5 if I bundle up inside. Toques are also highly recommended, and I bring a thin one no matter what time of year.

Like staehpj1, I have experienced crazy temperature swings in the mountains and I would never try to tempt fate by going without a sleeping bag.