I don't think splitting the calipers is usually considered part of a simple rebuild, that is why the seal between halves is not part of a regular rebuild kit. I suppose it does make putting the new pistons and seals in a little easier. I also suppose the seal between the halves could fail and cause a leak (making a case for replacing it), but I have never had it happen or even heard of it happening.
I had a heck of a time rebuilding the caliper on my TR250 (same caliper is used on a BJ8) spent a whole afternoon trying to get the dust seals on, the set I got were too big/thick, making it near impossible. I ordered another set, it was easier, but still involved a bit of time and effort and bad words to get them on. Go in thinking it will be hard, then you can be pleasantly surprised if it goes smoothly, better than the other way around....