The benefits of the jump cue is being light (with a stiff tip). As long as they are comfortable in the hand (balanced) while playing the length maters not.
What I prefer:
a) shaft alone is too short to be comfortable, but jumps well
b) play cue is too long and too heavy too back heavy, and is hard to jump
c) as light as you can get it with out it becomming overly nose heavy
d) rules require it to be at least 40" long, and this requires the butt to be at least 11" long (29" typical shaft) but is is best to err on the long side and make it 12" long (min).
Thus making the butt shorter to fit in the (esisting) bag is the antithesis of good cue design. The owner of a jump cue will buy his next bag so it has the space for the jump cue.