Obscurus is right.
Basically, it means that even if a person/group isn't making up "new" doctrines or spouting outright heresy, just focusing TOO MUCH on one doctrine can throw everything off balance and lead to error.
Focusing too much on the need for priests/ the Mass and you might be tempted to run a seminary that is utterly unfit and unworthy for the task, one that does more harm than good. Being downplayed is the fact that God will provide, and He will never require you to use evil means (pablo? dissimulation? deception? rogue pseudo-bishops?) to get that which we need (preists).
A similar point, equally interesting (that I learned in the seminary) was the dual need of
LIVES OF THE SAINTS
You need both.
If you focus too much on Lives of the Saints, you start to think that the extraordinary should be expected as ordinary. You also might focus too much on emotion and not enough on reason. You would be seeking out what kind of extraordinary saint you are going to be, rather than doing your boring old duty of state which is the ordinary means of salvation.
If you focus too much on doctrine, you forget that God is master of his gifts, and can make exceptions, visit people, give special graces, etc. In general, in this case you would be too cold, trying to be exclusively "reason" and forgetting emotion, charity, etc.