Pass rushers make CB's & Db's better. Without another reliable pass rusher the one's we have would most certainly wear down. Fortunately, in the Bears game Clark and Lowry provided a rush that might have otherwise allowed our DB's to be more exposed. QB's just aren't as good when they are getting pressure and that's Brady, Rodgers, or, whoever. Like MLF said these pass rushers don't often come available this time of year and I doubt Mercilus would be either if Houston hadn't switched to the 4-3.
This scenario reminds me of when Kampman became expendable for the Packers. Once the Packers committed to a change in D (I think in that case it was switching from a 4-3 to a 3-4) it was time to cut loose a well loved warrior.
"The train is leaving the station."