Maybe it's my more practiced jujitsu winning out over my less practiced FMA, but can you FMA guys shed some light on not securing the arm? Thanks.
I think that Tuhon Bill can answer this question better than I can, but as a starter:
Tapping is but a means to end, i.e., to familiarize the beginner with basic lines of attack and defense, build sensitivity, and to introduce body mechanics into muscle memory. One of the issues with empty hand vs. knife is that experienced knifers will typically feint and/or redirect an attack once the initial contact has been made - and tapping is designed to address that issue as well.
In actual practice (and one would pray that this would never actually happen to you) only a small piece of the drill would come out (say, a momentary check and redirect) and one would move quickly to neutralize the opponent before he can re-deploy the knife. Securing the arm is recognized as one of the best ways of doing this, whether as a momentary check, trap or lock, that could lead to some form of destruction or submission.
Don't take tapping qua drill in isolation therefore: it should be seen in the context of a wide array of follow-up techniques - many of which involve securing the arm. There is a nice discussion of this on the FMAtalk forum entitled "Knife Tapping."