It's a general rule, and it's mostly because I can tell when the chain's wearing down. If the belt is accessible, it's also not as big a deal.

As with all things, the build quality and overall design will overrule single atomized design decisions. GM iron dukes have geared cams, which should be frickin' bulletproof, but they use a nylon spindle that tends to blow apart under variable load or just using weird fluids.