Completion percentage has often been a simpleton's approach to quantifying a quarterback's accuracy. While we would still have to get some pretty advanced software to perfectly measure accuracy, we can do a better job of things just with game charting. For starters, not every pass is actually aimed with the intention of a completion. By breaking down every incompletion into classes, we can account for passes thrown away as well as drops.
Then there is the significance of how far the pass was thrown. If a quarterback wanted to complete 75 percent of his passes in a season, he could definitely hit that mark, but it would likely come at the expense of a productive offense.