Again, why bother directing the shield at them, unless they can attack back. . .forcing someone to declare their shield on you is as good as forcing someone to parry you for 20-25.
If they can't attack, go full OB, no parry, and declare the shield on someone else. . .
The roll referenced is to Crit AT 5 or 9 with a morning star (open the book and take a look), you need to roll over a 69 to AK those to ATs, a 30% chance. . .vs the 8% chance of fumbling the most fumbleable weapon in AL. You need 22 DB just to bring the two to parity odds.
That being my whole point. . . .
And you can keep circling around, but average fumble is less than 5, so the odds of open ending exceed average fumble odds. . .and 96+ results are usually quite juicy. (Average open ended up result is a 145). .That in and of itself is fully enough math to justify my point, without any of the rest. . .feel free to not roll, save some time. . .
But those of us who do roll are not merely showing our slavish devotion to the RAW, it's actually more of a dangerous attack than you choose to recognize.