Contract harvesting outfits replace their combines every season or every other season at, what, $200,000+ each. There's a tax advantage in depreciating the equipment quickly and they don't have to take quite so much chance of downtime vs. older equipment. That makes business sense; if you're a really big farming operation, you probably also do that with other big ticket items, like tractors, spray rigs, planters, etc.
If you're not farming on such a grand scale, though, it's hard to see how frequent trades pay off. And for rank amateurs like me, the distinction between "needing" a new tractor vs. "wanting" a new one gets kind of fuzzy.