I can see it possibly being useful in a situation like the following(which may be completely off, as I'm still digging my way through A6):
class Foo { method bar is public is rw {
} }
Becoming:
class Foo { method bar is public & rw {
} }
Guess it just reads a bit better to me.
And you might even be able to do some weird stuff like:
class Foo { method bar is public | rw {
} }
Whereby bar is only an lvalue subroutine/method internally.