The standard shunt-type regulator runs the stator at full belt all the time and shunts back the unused current to the stator windings, by a clever design utilising the characteristics of a three-phase outlay. Unfortunately, this means the stator has to deal with a triple-whammy of heat.
1. The heat of the engine oil
2. The heat of generating power at 100%
3. The heat of shunted power being shoved back into it.
The shunt regulator was an answer to a problem that Suzuki had at the time -how to use a cheaply-made PM alternator and control it with current consumer grade electronics. Anything fancier would have been expensive and way over the needs of the market for these bikes.
The shunt regulator answered their problem - for a while. It got the bikes off the showroom floor and lasted long enough to get the bikes out of the warranty period. If the subsequent owner was lucky, it lasted him, too. The third and remaining owners usually weren't lucky. By the time a few years passed, the connections in the charging system were showing their age and started to overheat, leading to the demise of the battery as the reg-rec tried to overcharge it and then the regulator would die often taking the stator with it.
This is why Suzuki only ran the bike on two of the three stator windings when the headlight was off, and why the third winding came in with the headlamp switch. It was an attempt to limit generated heat load on the windings. It worked, to an extent, but wasn't guaranteed to last.
However, there are some owners who've always run with a headlight load (daytime headlights in the US, for example) and they've been lucky to escape terminal stator and regulator damage.
All that is now solved with the advent of the series regulator, which simply cuts off the charging current at the voltage set-point (14.5V) and the inbuilt quenching and damping arrangements of the reg-rec deal with back emf and transients. These power electronics weren't available at any sort of sensible, reliable price or specification forty years ago.
I've been using a Shindengen SH-775 for several years now and just for gits and shiggles, I've deliberately been running an old stator that would have died by now if it had still been coupled to a conventional shunt regulator. It's surviving fine, so far, after about 20K miles.
The beauty of the series regulator is that I can treat the stator just like a field-coil generator and leave it unloaded, or put whatever loads I like onto it (subject to its max, of course) without worrrying about whether it's going to burn out today or next week.