Cancels in DMC4 is mostly situational other DT than Vergil and Lady who have a lot of usage in combat.
Doing multiple Rapid Slash in DT enables you to not only move faster but also skip certain fight triggers like Alto Angelo intro in 9.
General carryover charge method is to release shoot, then switch weapon closely after. Add in a jump if necessary.
JC is part of the upgrade shop and it costs heavy proud soul to obtain.
DT Cancel allows Nero to cancel the recovery off the move, get some free i-frames, or recover from getting hit.
iFrames example against Agnus spinning blade.