1) All he's done is benefit West Ham. Since coming on in midseason two years ago, in the midst of the dreariest season for the Irons in recent memory, he's saved them in the most improbable way imaginable and then guided them to a comfortable mid-table finish.
If somebody had asked me two Decembers ago if I would be content with finishing 10th in the Premiership the following season, I would have responded enthusiastically in the affirmative. I doubt that I am the only one who would have done so.
But since the football his teams have played is often basic, dull, and boring, when the results don't happen, the cries for his head are louder than they would be for a manager who inspired his team to play more imaginative and exciting football.
2) He's been forced to employ a squad that has very few of his own additions in it. All of Curbs' major signings of last summer—Dyer, Bellamy, Faubert—were injured for all or large parts of the last campaign. He's used Alan Pardew's players of choice much more efficiently than Pards ever did, and squeezed much more class from them than he had any right to.
While it maybe wasn't entirely unexpected that these players suffered injuries given their medical history, I doubt anybody thought that they would all miss quite as much time as they did. Curbishley deserves at least one more season to demonstrate what he can do with a full side of his own players.
3) Patience is a virtue. While football fans are notoriously short with their managers and clubs have been known to be a bit hasty to give out the sack, the fact remains that one full season isn't enough to fully evaluate the man as a manager.
Even if he hadn't been dealing with an injury list longer than some newspapers, even if he hadn't often put the fans asleep in the seats, Curbs deserves at least one more season to show the West Ham faithful what he can do before they begin searching for a new boss to lead them to Europe.
Building a top-level program takes time, regardless of who's in charge: stability in the club won't ever be a realistic goal if a new manager, with a new philosophy and a new strategy, is brought in every 18 months.