There's a big difference between doing a test that is basically structured (sensei calls out waza) with some ji-waza thrown in and doing a test that is basically free form (please show 5 techniques from each of the basic attacks). In the more traditional, structured style, you only do ji-waza from one or two attacks and it doesn't really make sense to prepare a script for each and every possibility. Most of the test is scripted anyway (just not by you) so it's fine to leave a little room for creativity.
In the free-form style, there is a real fear that without some form of preparation you will end up doing the same techniques from each attack over and over again, and that you may forget to show some of the more elaborate techniques that are expected on a shodan exame. Not that either of these possibilities is so bad, but as you come up for your test they can really trouble you (or at least me). That's where the urge to plan comes in.