Is there some confusion as to what a toothed belt is?
A segmented V-belt would be a strange choice for a generator drive belt. Segmented V-belts (otherwise known as linked belts) as shown below were traditionally found as drive belts on ancient bikes that didn't have gearboxes. They were originally made from leather segments that link together. I guess rubber V-belt technology was in its infancy back then.
They're still used in some light-duty applications where the adjustable length may be useful for some reason, but nowadays they're made of some kind of reinforced synthetic material.
For a given pully groove size, a segmented V-belt is not as strong or durable as a reinforced continuous rubber belt, so would generally be an inferior option for power transmission.
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I suppose it's possible that Paul has a segmented generator belt, but I suspect what he's talking about is a continuous rubber belt with "teeth" moulded into the inner edge (shown at left below), but they're not actually teeth in the sense of the toothed timing belt that Piet has fitted. They are only toothed in as much as a wavy brake disk can be described as a having teeth. It might make more sense to think of it as a notched belt rather than toothed. The notches don't engage with anything. They're only there to make the belt more flexible. The toothed V-belt still relies on friction between the sides of the belt and the V-groove in the pulley for its drive, so it can slip just as a plain V-belt will if not tensioned properly. Note that the inside edge of a V-belt (whether toothed or not) does not contact the bottom of the groove in the pully. If it's worn so much that it does, then it's time to replace the belt.
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The flexibility of the toothed V-belt allows it to conform better to the curve of the pulley, especially with small diameter pulleys, giving better grip than a plain belt.
The toothed V-belt also has the benefit of running cooler than the plain type. Without getting into V-belt drive theory or hysteresis of rubber deformation, suffice to say that rubber absorbs energy as it bends. That absorbed energy heats up the belt. A belt that bends more easily as it rotates around the pulleys will absorb less energy and will therefore run cooler. Heating up the belt wastes energy and accelerates degradation of the rubber, so a cooler belt is more efficient and will last longer.
It's been my experience that V-belts available in automotive shops these days are predominantly the toothed type. It's getting harder to find the plain type. Not that it's a problem as they fit the same pulleys.
Piet's flat toothed belt drive is a different thing altogether and has a lot of advantages over a V belt. It's basically a timing belt. The teeth engage with those in the pulleys (sprockets may be a better word), giving a positive drive, much like a chain. Timing belts require less tension than V-belts because they don't depend on friction to provide grip between the belt and the sprockets. This leads to lower bearing loads and lower wear rate due to reduced stress within the belt. They're pretty much maintenance-free in that they don't need to be periodically tightened like V-belts to maintain optimum drive. The initial tension will last for the life of the belt. Timing belts are thinner and more flexible than V-belts, so are even less prone to thermal failure than any kind of V-belt (toothed or plain). The only minor disadvantages of timing belts are that the pulleys are a little more costly to produce, and they're more sensitive to misalignment than V-belts.
I replaced the plain V-belts (water pump/fan, alternator and aircon) on my car with the toothed type about a year ago, for the simple reason that they were the only type I could find. It wasn't a conscious decision. They ran OK for around100 km then started slipping like crazy, making the typical screeching noise. I re-tightened them, and they ran for about 1000 km before slipping again. I was starting to think I'd bought rubbish belts, but I tightened them again and they've been OK ever since. I guess they just took a while to stop stretching and bed in. I've not noticed such an obvious running-in period with plain v-belts before.
PS: I don't know much about the Laverda twins, but all of the above is just general belt drive info. It's not specific to the generator belt of the Laverda 750.
