Unless most of the teams in your league carry a backup defense, more than half of the league should be available to you every week. Perhaps instead of playing one above average defense every week, regardless of matchup, we should instead target the widely available waiver wire defenses with the best matchups in any particular week? You can always make exceptions if your rent-a-defense gets hot - heck, you might find a team worth starting all year by accident. Even if you don't, good matchups will continue to get greater in number as more QBs get injured or yanked as the season goes on. You can steal some points from bad weather come December. Each week I'll list the startable fantasy defenses on most waiver wires and rank them.
**New England and Chicago have great matchups in Week 3 but are assumed to be rostered**
Week 2 Results
New England (at Miami) - 7 sacks, 4 INT, 2 DEF TD, 0 pts allowed
An orgy of D/ST production. Congrats if you got the Patriots they might help you win your league