I see where you're coming from and the Jets definitely have a scary D. However, as the first poster said whether you feel you are losing "double" by putting him in really doesn't matter even if it's true. At the end of the day the quarterback who scored you the most points that week wins no matter how you look at it.
For example if Flacco got you 20pts but threw 4 picks to your opponents defense you still got 20 points.
Same scenario again but you start Cassel. He scores you 15pts but Flacco is still gonna play in that game vs the Jets and throw those 4 picks. You still lose out on 5pts.
Your thought process makes sense but it's not sound.
IMO Flacco will outperform his ranking for the week. The Jets D is good but there are so many weapons for Flacco to hit I don't think it will matter.
10 Team QB Flex 1PPR - Week 3
QB: M. Stafford, J. Flacco
RB: C. Johnson, D. Murray, D. Sproles, CJ Spiller, M. Leshoure
WR: J. Jones, E. Decker, A. Brown, P. Garcon, B. Lafell
TE: J. Finley, M. Bennett
K: D. Akers