From the Pro-Football Reference Blog:
[Imagine w]e have two teams that both average a whopping 14 yards per attempt. One team completes 100% of its passes; the other 50% (for 28 yards per completion). If I were to model those two, it seems pretty clear that the team that completes 100% would score more. They would score on virtually every possession, only failing to score in limited cases where their 3 consecutive completions net 9 or fewer yards. The 50% team would also score a lot, but string together a few more droughts. I suspect my 100% completion team with 14 yards per attempt would average about 60 points a game, while the 50% completion team would average closer to 50.
At the other end of the spectrum, we have two teams that average 3 yards per [pass] attempt. If one of those teams completed 100% of their passes, they would struggle to maintain drives or even get them started [or would routinely end on 4th and 1…], while a 25% completion team would occasionally string together first downs and get into scoring range. Neither would score much at all, but if I were forced to watch both teams for 24 hours straight as punishment for all my transgressions, I’d take the team with the yards per completion to win in a non-shootout.
And later in the piece:
[I]t is pretty clear that the QB’s in a Don Coryell-based offense (Fouts, all of the Redskins QBs of the 1980’s and early 1990’s, and the Rams and Chiefs recently) are undersold by passer rating relative to adjusted net yards per attempt in terms of the value they provided, and the West Coast passers are oversold, and its because of the different philosophies as they affect completion percentage.