Yes its very nice !
Beside the quite obvious use cases that are given by the standard library, I have seen that feature used to add locations to anything this way :
%public %inline located(X):
x=X { Position.with_poss $startpos $endpos x }
After that, in every rule you can use located(expr) if you want an expr with its position added. Obviously the ocaml code here may vary, this is why this is not part of the stdlib.