And yes of course I consider my example to pass $#a as an argument an extremely common case.

The validity of your example as a counter-argument relies not just upon passing $#a as an argument to a sub, but also on:

1. Not assigning the arguments to named lexicals within the sub as is normal:

   sub ...{
       my( ..., ... ) = @_;
   }
   [download]

2. And then, using the $_[...] alias as an lvalue.

I bet you cannot find a single example of anyone doing this on cpan; or in any publically available piece of real code.

Ergo, not a common case.