Errata is very important, but not always available or relevant.

What do you do if you design with chips that will go in production 9-12 months from now? The current errata shows the state of early spins. To be competitive, you often have to put a lot of trust in a manufacturer, which means that it can be more interesting to look at errata for previous chip generations, to see the types of errata that gets solved (and how quick) and the types of errata that they permanently leave unfixed.

Few chips are so simple as the 8051 family - with complexity comes problems.