How important is the defense to your team? The simple answer is not very important relative to your positional players, but a good defense can be the difference between winning and losing a week. On average a defense will earn 5-10 points, anything over 10 points I consider to be great. Almost always you get points for sacks, ints and fumble recovery and some leagues get points based on how few points and yards they allowed.
When evaluating the football teams defense it is important to understand their team as a whole. One trend I noticed is when a team has an offense that scores a ton of points, then their defense will often let up alot of points. Usually in the NFL you don't see games for 41-7, if one team is dominant the winning team's defense usually plays less conservative. Now this can be a good and bad thing, so let me explain. If your defense is giving up tons of points thats bad of course, but if they are forcing the opponent to force alot of passes it creates the opportunity for turnovers.
Simple logic of injuries also apply to defenses. If a team's star cornerback is hurt, thats would obviously be a red flag. Now there is a debate on whether or not you should have two defenses on your team week to week. The advantage is so that you can play the matchups correctly. This is a very good idea because like I said in my previous matchup post, playing them correctly can net you a ton of points. I would only keep one defense on my team if it was one of the few elite defenses in the league.