^^ says "focus my target if it can be attacked *and* if it exists". If your target can be attacked (if [harm] is true), it exists. The exists condition does nothing for you in this case - it is implicitly implied with the harm condition.
It's not hurting you. But it's not helping you, either.
/edit: if you have no target, then [harm] will return false (because you can't attack a target that does not exist), and /focus will not be called.
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh