[investigator2100 (OP)]

Permanent magnet motors: why do we use the pairs of unlike-poles of the magnets instead of like-poles?
Pairs of unlike poles of the permanent magnet can produce only attraction force.
A single magnetic force can act as a spring force only. Attraction force can attract an object only then the movement ceases. Repulsion force can repel two magnets away then the magnets cease to move. So, none of both forces can alone produce continuous motion.
The alternating occurrence of both forces is must between the same magnets to produce continuous motion; production of continuous motion is must to do work continuously.
If a single pole of a permanent magnet is used (instead of a pair of unlike poles) against an armature tooth the permanent magnet pole will produce both attraction and repulsion forces in harmony with the attraction and repulsion forces created by the armature teeth. The single permanent magnet pole will not have to attract the other member of an unlike-pole pair, so all its magnetic flux lines ‘B’ will have to act with the armature teeth only.
No repulsion force is produced in the usual PM DC motors. The armature teeth always travel to stronger points of attraction from weaker points of attraction. So, these motors work on the resultant attraction force. Repulsion force is totally wasted in these motors.
Why should the use of both forces that are created by a single PM Pole, not make a motor more efficient?
The use of single-pole does not mean that only one permanent magnet pole can be used against a whole armature. Instead, many like poles can be used against an armature.
Every one of the like poles will act independently with the armature teeth. So every one of the like poles will exert both attraction and repulsion forces on the teeth if the teeth are arranged properly.
Next time I will name the single-pole configuration as mono-field-poles configuration.
Link to the patterns of B lines of both Kinds of field poles is attached

[Bored chemist]

Permanent magnet motors: why do we use the pairs of unlike-poles of the magnets instead of like-poles?

We do that because it works.
Reversing one of the magnets would stop it working.

Of course, not all motors actually use two magnets, so the point is moot.