Just an FYI, ImageX.exe is still an included and used tool in the WAIK 2.0 (7/R2 version) and is intended to be used in WinPE 3.0 for deployment scenarios.
ImageX - can still be used to mount, and commit changes to a WIM file.
DISM - is the "preferred" method because of the ability to remove package files, drivers
, language packs, etc on an offline or online wim image. Can mount and unmount WIMs.
PEimg.exe - deprecated in WAIK 2.0 (previously used to load drivers to an offline WIM image)
drvload.exe - deprecated in WAIK 2.0 (previously used to load drivers to an online instance of WinPE 2.0 "on-the-fly")
You cant APPLY or CAPTURE a WIM with DISM.exe, so ImageX is still required and useful.