The benefit of a field coil alternator is that it makes voltage regulation relatively easy, you control the field coil current and that directly controls the output power, so the system can control the battery voltage to keep it charged and avoid boiling it dry.
If you don't want electric start you can skip the battery and use magneto ignition plus an AC lighting power alternator. Most old two strokes work this way.
A permanent magnet alternator/dynamo/generator produces full power all the time.
The bike designers can exactly (approximately) match the alternator output to the load power - that means extra output coils switched in when the lights are turned on: 60's Japanese bikes did this pretty well but old Italian bikes are infamous for the bizarre switchgear and wiring nightmares they managed the job with. This approach requires that the electrical load stays the same, no extra lights and such are allowable. Plus the match is never exact and batteries rarely last more than a season.
One can dump the extra power into a heater, Joe Lucas liked this approach via a simple 14V zener diode on a big heat sink: Honda does much the same (a bit more elegantly) on some GL models. This sinks engine horsepower into heat though. The GL1100 had a few HP to spare and was not a sportish model anyway, Honda didn't care about the weight, cost, or wasted power.
PM rotors are heavier and more expensive to make than the field coil type, plus in the 70's some metals and minerals used in high strength magnets got very expensive.
When making the CB fours apparently they did care about lost engine power and extra weight.
The induced field system (as pictured) avoids the complex, costly, and troublesome brushes and slip rings but is less efficient at turning electrical power into the necessary rotating magnetic fields. The alternator structure can be thinner too, no room needed for brushes.
