I still like Carter better, but having both gives us the advantage of having two very good 3-4 DE's and all of the sudden we have the ability to pull Jason Taylor and David Bowens back into 3-4 OLB positions. Junior Seau has experience manning the ILB spot in the 3-4, and Zach would be the question mark.
You take Larry Chester and Tim Bowens rotating at the NT spot, add an Anthony Simmons to the mix at linebacker for rotation purposes (to make sure we're not depending on JT and D-Bo as starting every down 3-4 OLBs) and all of the sudden IMO we have ourselves the 3-4 personnel that everyone claimed would be impossible to put together in Saban's first year.
Add that to the fact that in the 4-3 alignment we can put JT back at his more natural RDE position, rotate Vonnie Holliday and Kevin Carter at LDE, with Larry Chester, Tim Bowens and another DT yet to be named (or drafted, rather), and you have Zach to man the MLB spot, an Anthony Simmons or Donnie Spragan or Matt Stewart to man the SSLB spot (or WSLB depending on whether it is Simmons or Spragan/Stewart) and Junior Seau.
Overall add all these names and I'm seeing a lot of high quality rotation, and a lot of 3-4 alignments that KB21 was sooooo sure would never materialize.