The Bane was SF's first attempt at a curved hilt, and I think they did a pretty good job, considering the complexity of the design elements. That said, the Vanquish showed that they are ever honing their craft, as it showed improvements in the areas you mentioned. Also, it was announced awhile back that there will be a v2.0 Bane that will incorporate those changes, so keep watching!
All that said, my son has a Bane, and while the seam is noticeable if you are looking for it, it is not so wide that it grabs all the attention from the other wonderful aspects of this saber. There have been cases where the assembly varied a little from hilt to hilt, leaving some with a more noticeable seam than is ordinarily the case. In these cases, a little disassembly showed that there was a wire, or some other small object caught between the pieces keeping them from seating correctly, and reassembling with that situation taken care of produced a much better effect.
I cannot recommend doing this yourself if you are not familiar with these techniques, because doing so would almost certainly void your warranty. However, if you saber is still under warranty, and you feel this might be the case with it, contact Customer Service (This email address is being protected from spambots. You need JavaScript enabled to view it.
), and ask if it would be possible to RMA it to have it checked out. Likely, they would want to see pictures to determine if this is a normal seam, or an exaggerated one that warrants an RMA.