I once again find myself in disagreement with other advice givers. While I would hang onto the Bears for the two Detroit matchups (cha-ching!), they otherwise play a bunch of high powered offenses and will get totally smoked as a fantasy defense, at least in the nfl.com scoring system (which I suspect the original poster is in).
Interceptions (especially for TDs) are very important for fantasy defenses. Yardage doesn't usually amount to squat, when you figure that at 300 yards you get 0 points, and 300 yards is not difficult for most offenses to obtain. Low scores help, but if anything we've seen a trend towards high scores.
Defensively, the Bears have only had 1 good outing in 3 weeks from a fantasy perspective, and that was still much worse than what lower-ranked defenses have produced.
I say, if you can swap a WR (WR's are a dime a dozen ... there are great ones in all of my free agent pools, many who should be starting for other players) for a defense with a good, aggressive secondary, do it. Look for good secondaries and good special teams play. Even the worst defense has a better chance if they have more field to defend, or if their return guy scores a TD once in awhile. A few defenses haven't emerged yet this year. If you can guess which ones, you may take home the prize.