Before the season, the Rams didn't think that Clemens could get the job done as a backup; the job was given to 2nd year player Austin Davis. However, after Davis had a bad preseason, the Rams decided to cut him and keep Clemens as the backup. At worse, the Rams envisioned Clemens starting for a couple of games. He's been with offensive coordinator Brian Schottenheimer for years so they had familiarity, and he would help Bradford out in the film room.
Clemens hasn't set the world on fire this season, throwing for 634 yards, 3 touchdowns and 2 interceptions, plus one a fumble. Not horrible, but it's good enough. He threw 2 interceptions against a talented Seattle Seahawks defense, and against the Tennessee Titans he fumbled on a key offensive possession. Also, needing Clemens to throw a touchdown to win a game is like trying to win the lottery: you think there's a chance and then you lose.
But he does what teams expect backups to do when they have to start; just enough so that play makers can win it for them. There's 6 games left of the season. If Clemens continues his solid play, there's really no reason the Rams should acquire another backup unless they want to go for a younger player.