IMO the switch in defense is more about the "Base" front, instead of starting out in a 3-4 front when opposing offenses come out in their "base" offensive formation we will show a 4-3 front.
The pros and cons of a 3-4 front - 3 hefty lineman to impact running lanes and take up double teams to open up blitzing lanes for the 4 LB's. If those 4 LB's cant shoot the gaps effectively and squeeze by occupied blocks the effectiveness is hindered leaving our secondary vulerable in extended coverage.
Side note - the LB's won't always be blitzing, the defensive call may have 2 or even 3 Lb's back in coverage or vise versa coming with pressure.
The pros and cons of a 4-3 front - 2 hefty DT's to impact middle running lanes and collapse the pocket right up the middle forcing the QB/RB to change direction to the outside. This would allow the DE's and OLB's to clean up and leave our MLB clean to roam and attack any and everything. If the front 7 (DL and LB's) don't create pressure or disrupt the offense it leaves our secondary vulnerable in extended coverage.
Side note - the 4-3 front requires more agressive run support out of your 3 LB's due to the DE's primarly taking a wide pass rush angle, in most case's this stress more support from the SS leaving the FS to roam deep thirds. The Strong side OLB will always line up over the TE, if the RDE is left 1 on 1 with the LT he should be able to impact the LT and allow the weak side OLB and MLB enough time to flow to the play and clean up. The RDE responsibility is to keep the play INSIDE where his help will flow to and take care of the play, not stop the run.