This answers Johannes's objection : a malefic in a good house, not dignified, can bring good.
This may or may not be an accurate description of Morin's views.
I cannot debate the ins and outs of Morin's perspective like Tom who is a specialist in that area. However, I cannot accept what you are stating is mainstream traditional astrology. Lets not forget Morin was quite idiosyncratic on a number of points! Historically, he stands at the end of the traditional era and took issue with many fundamental concepts in medieval astrology.
I suppose it depends what you count as a 'good house'. A good house for a malefic without any essential dignity and especially if debiliated by dignity is probably a cadent house where it can do less harm! A malefic in a strong angular house is more powerful but not more benefic because of this power. In fact it can be very damaging.
In a horary or electional chart the last thing you want to see is an angular malefic without essential dignity!
There are of course many mitigations. The condition of its dispositor, a positive planetary phase, sect, aspects to benefics, or benefic fixed stars etc. Still, I cannot see how this changes an essentially malefic planet lacking dignity into a benefic. They just lessen its ability to cause damage. Having a malefic lacking any dignity in a powerful house simply increases its potential to cause harm unless there are other mitigations to this.
In essence I agree with Johannes that essential dignity by sign describes how a planet expresses its basic nature for good or ill. Accidental dignity , such as house placement is about how powerfully it will be able to express that fundamental nature.
At least thats the normal viewpoint. However, its interesting whether the relationship to the Sun by sect or phase can actually alter the nature of a planet. Reading some hellenistic sources sect appears to fundamentally alter the way a malefic expresses itself due to its being in or out of planetary sect. Saturn is certainly less malefic in influence in a day chart and in its oriental phase to the Sun. However, I dont think house placement ever changes the malefic nature of a planet.
Robert Schmidt has suggested that rather than dividing significators into essential and accidental dignity the hellenistic astrologers looked at three basic considerations:
Zodiacal -including dignities, aspects, fixed stars etc
Heliacal-relationship to sun ie planetary phase and sect
Mundane-relation to the earth ie houses
Mark