In these schemes, the common theme is the replacement of DEs with rush linebackers. Thus, the acquisition of Lee, Carter and Connor to go along with the retainment of Butler and Spencer should make more sense. Having 5 or sometimes 6 LBs on the field at one time is something like the Giants use of 4 DEs (instead of a traditional 4-3 front that has 2 DTs) on passing downs. With all the pass rush capability lined up all over the place, the defense is keen on confusing the QB and overloading one side of the O line with pass rushers.
Spencer's replacement isn't one specific person - it's a group of capable LBs (Carter, Butler and Connor) that all have their strenghts and weaknesses but as a group utilized via the above schemes, can and might equate to the abilities of Spencer PLUS add an aspect of pass rushing which he lacks.
Retaining Spencer for this year, just one year, is buying time to allow secondary upgrades to learn the new scheme and to give newly acquired LB talent an opportunity to thrive. Both are dependent on the other: better LB play can be cancelled by lackluster press coverage and vice versa.