Here's the issue that is not being distinguished - the time factor. For example, how many councils have we had in the past and how many doctrines have they defined? (at least 20 councils). And how many Doctors of the Church and saints and other holy people, including popes, have written and explained such doctrines? Hundreds. So, the previous magisterium's of the Church have already "spoken" and already explained all that's needed to be said concerning most of these doctrines. For example, the doctrine of Christ having 2 natures - divine and human. This has been around and explained for so long that we catholics in the 19th/20th/21st centuries don't need it re-explained. It's pretty basic.
So, such doctrines are settled. The current magisterium isn't spending time re-explaining these types of topics because the Church has had 17 centuries to do so. So, to say that the current magisterium is the 'rule' of faith, is not accurate for this topic. It might be accurate for issues which are CURRENTLY BEING ATTACKED or which need to be clarified, but for older doctrines, we look to the past for explanations. We can do this because the Church's teachings are CONSISTENT and UNIVERSAL. Therefore, what She said 16-17 centuries ago concerning Our Lord's Divine nature was as accurate then as it is now.
The point is, to say that the current magisterium is the 'rule' of faith is only partially correct. The magisteriums of the past (i.e. previously defined doctrine/dogma and the related commentary) is part of the 'rule' of faith as well, because Church Teaching is eternal, no matter what time period it came from. So the rule of faith is THE MAGISTERIUM (past and present) because ultimately what they teach is ETERNAL TRUTH, which is timeless.
So, to Drew's point, whether or not the current pope strays from the Faith is irrelevent to our Faith because the current magisterium is only a small part of The UNIVERSAL magisterium, which is the constant teaching of the faith over 2,000 centuries. This UNVERSAL magisterium is what Drew means when he says 'doctrine'.