I am lost on one point of the original concept of the thread. "Will BIAB help me play jazz better".

The effective answer to that question is "No. Nothing will help YOU play better but a lot of practice."

What BIAB CAN do is give you backing tracks to play AGAINST and make the process of creating those tracks much easier than a regular music sequencing recording program. With Cubase, Sonar (what I use), Pro Tools, whatever, you still have to PLAY G-2-3-4 G-2-3-4 Em-2-3-4 Em-2-3-4 C-2-3-4 C-2-3-4 D-2-3-4 D-2-3-4. In BIAB you just have to TYPE in the chord names G EM C D in the right place and BIAB will create a backing track with that progression.

"Jazz" really has nothing to do with it. Nor would pop, rock, funk, soul, country.... You just pick an appropriate style for BIAB to emulate when you create the backing tracks and go for it. When you get done with that key, transpose it up a step and play it in A for a while. Then maybe move it to D so you practice in every key. Change styles to play with different grooves, change tempos to play different speeds.....

Maybe your root question is better stated "Will this give me music to spend hours playing along with and thus improving my skills". Then the answer is yes. However, another option is to play CDs of your favorite artists and play along with them if that's what you are looking to do.

Most people I know use this as a writing tool and a backup band for composition. I just finished my 9th song for a CD I am trying to do. I use BIAB to make home studio quality demos to send to the players who will do the CD so they can learn the songs before we waste a lot of rehearsal (or studio) time teaching parts. Writers need to spend their time writing, not teaching parts.