I don't see how it rewards unscripted teams. You just create more scripts to accommodate more possible witness lineups. If a team has to adjust "on the fly", that means they didn't think through and prepare for all of the possibilities. I also don't place much value on "stealing" witnesses, at least to the extent where the vast majority of the affidavit clearly favors one side. A plaintiff-oriented witness testifying for the defense (or vice versa) and trying to spin inconvenient facts from the affidavit usually comes off looking pretty foolish in my humble opinion. One of my big mock trial pet peeves is the abundance of teams who seem to find value in introducing evidence for the sole purpose of discrediting it.
First of all, I'm still a bit confused as to what constitutes a "script" vs. having a blueprint for a direct. Is it just the extent of the written words on the page you practice from, or does it imply some author outside the attorney-witness pair? Are there really any [good] teams that have no idea of what they're going say or how they're going to say it when they take the stand? The whole discussion of "scripts" seems to be some sort of insult to certain teams (at least theoretically), but I seriously don't know who doesn't rehearse directs and, particularly as the season progresses, have at least a "groove" of what they say and how they say it.
With respect to "stealing" witnesses, I think it's just a part of the game. I don't think the reason to do it, however, is to discredit the testimony from the witness' affidavit, but rather simply to emphasize other parts of it. Mock trial cases are the furthest thing from reality (shocker, i know

) with no objective truth for any of the witnesses. Half the fun is spinning the ridiculousness that is in some of these affidavits into some sort of real persona with a real perspective. So much of what we get is up to interpretation, and I think being able to realistically portray a witness on one side who generally is called on the other has a certain value in this activity.
Of course, there are limits, and certain witnesses can never be called by the other side without it being farcical. Also, the benefits of preventing the other side from calling a witness (along with the positive facts in their testimony) must be weighed against the costs of not calling a more obvious witness for that side. It's clearly a judgment call.
And yes, this does create the need for many more backup crosses. Again, such is (or was prior to this past year) the nature of the activity. Again, just because something is tenuously prepared beforehand does not mean there's no thinking "on the fly." Not only do you have to be able to learn/edit/rewrite a cross on a moment's (or case-in-chief's) notice, but you obviously have to be able to adjust all parts of your case in order to accommodate any changes.