Most, like Wil, do the right thing. Delicious Library hasn't had a major release in years, but it is maintained.
Others (for example, in my experience "DVD Remaster Pro" and Parallels Desktop), clearly abuse the major update system as a revenue source. They release fake major releases with lots of new skin but few features every six months or a year to rake in the dough.
Honestly, it's a mess. But the solution to me is obvious: Angry Birds from Rovio. Rovio didn't just release the thing and stop. They keep adding significant features (the game is probably 10X as large as when I bought it for the iPad). Customers feed back with great reviews. Rovio is rewarded with continuing sales and stays in the top seller lists. Meanwhile, apps like "Plants v. Zombies" are static and drop off the top lists quickly after a quick burst of popularity.