If they are not being crudded up by deposits and are maintaining their gap,
then there is little reason to do other than replace them at the interval
recommended in the owner's manual. If I had one of the four-bangers with two
plugs per cylinder, I might tend to inspect more often than the manual
recommends.

In general, no.

Spark plug life depends on many things, including type of spark
plug (electrode material), spark plug heat range, engine oil
consumption and type, type of driving, condition of the
ignition leads, and milage since last changed.  They do /not/
simply deteriorate with time.

Maybe someone's circumstances were such that an annual change
was optimal by chance.

If you're using an oil specification recommended by the vehicle
manufacturer, consumption is within manufacturer limits (might
be a lot higher than you think), and the vehicle is well
maintained in other respects, then the recommended spark plugs
for your driving type (stop-start, freeway, etc.), should
continue to last the recommended milage.

John