Can't have everything . . . the cam pinion teeth will indeed always be loaded at the same points - as the lobes whack the followers. But if the idler teeth which bear on the cam pinion are constantly changing because of the idler's funny size, then the overall wear on the gear train will be much less than if it was the same idler tooth contacting the same cam pinion tooth every time for each follower. That's the theory anyway. BSA, Triumph and AMC twins followed the same practice in their different ways, successfully I'd say. Norton and Royal Enfield twins of the same era preferred chains for their single and twin camshafts respectively (hence their clockwise rotating magnetos - no reversal of direction en route).
Not sure whether chains/sprockets suffer from localised wear or not (suppose they must to some extent), but they are certainly more of a pain to set up than a gear train, there are 2 of them (one for the mag/distributor/whatever), and then they need periodic maintenance as well to keep them tensioned. And that's without thinking of how to turn the dynamo, if there is one.
Cars are pretty much the same: setting chain driven twin ohc engines is a fiddle; gear driven is in principle a lot simpler - although as anyone will know who's played with a set of helical timing gears, they can be a fiddle too.
And while we're on, why did only AMC have the wit to use steel mag pinions? I have now seen no fewer than 4 fibre mag pinions fail in the last 18 months or so: my A, a Sprung Hub Speed Twin, a Vincent Twin and a Venom. That's a high rate of attrition, albeit on some very well-used bikes. The A and a local friend's Speed Twin now have alloy gears. Now, right back on point, they probably DO need swapping around on their shafts from time to time to even out the wear!
