When I'm actively running (calling CQ) and someone sends out QRL? , my reponse will vary:
If it comes at a slow point, I will reply with QRL QRL just to occupy the freq longer and make sure I have the high ground if a frequency fight ensues. Agree with all Q codes have an interrogative (Are you?) form and a declarative form (I am.)
If it comes at a high rate point, I'll usually just send YES and continue sending.
If the QRLer starts CQing after that, we then move into frequency fight tactics discussion, much of which depends on how loud and/or score conscious you are!
73 John K3TN