It's a problem when it comes at the expense of the job's versatility. Were the idea more like a RDM samba-esque effect where anyone hitting the same mob they are gets the 15% benefit, rock on, but as a standalone spell, eh... Keep in mind I'm speaking to try and offer incentives to that concept of versatility, not seesaw things one way just because we happen to have "mage" in our job name.