[greymule]

why not just say (loudly) HE MISSED FIRST as you tag him?

Yes, that would certainly do it. I didn't even think of that.

There are some weird scenarios in which a runner, to prevent a run-nullifying third-out appeal, could make a deliberate third out by running directly into the dugout or by catching a throw or something, but that kind of oddness arises with other rules, too. My main question about this rule is, Why? How does it improve on the old rule? What could have happened to make somebody even think of creating a rule different from everybody else's?

Years ago, MLB handed down a ruling on the following play (using softball notation):

R1 on 3B, R2 on 2B, two out. B3 hits a ground ball to F5, who instead of throwing to 1B opts to tag R2, who appears to be giving himself up. But R2 reverses direction. F5 chases R2 for a couple of steps as R1 crosses the plate. F5 tags R2 for the third out but then throws to 1B in time to get B3.

I guess that because this play did not involve a base missed or left too soon, there was some uncertainty about whether the out on B3 at 1B was obtainable by appeal, and even about whether such an out actually constituted an appeal. So MLB ruled that a fourth out can be obtained by normal play on a force or on the BR before reaching 1B. ASA has made it clear that such a fourth out cannot be obtained, and in fact uses in its case book example the BR out before reaching 1B.