I tend to agree, I know I'd be way too uncomfortable in a -20 C bag in most conditions assuming I had a good mattress/mattress system. However, its also a personal thing, my kids seem to do just fine in bags rated much lower than the actual conditions.

Actually, as an interesting aside, I used to camp even in the summer with my -7 bag and a low-quality mattress (I didn't do any winter camping back then). When I finally got a good mattress (R value of around 3.5) I found I was routinely too warm. Ultimately I ended up getting a +5 summer bag that I now use unless the overnight temp is actually forecasted to drop below that. I've found the same with my winter set up, unless it is really going to drop below -10 or so the full-blown bag + overbag can get too warm. On the flip side, if the temp is going below what I am equipped for bag wise, extra (dry) layers can help fill the gap (in one case, even partially zipping my down jacket around my bag was helpful).