As a frost mage, and a consistent top performer.. I am in the first vision group because I have the ability to not only tunnel boss when clear, but when I get FoF proc, my ice lance one-shots the bloods (thanks also to archi trinket). If mages are not going into the vision, either there is something wrong with the mages play, or guilds have different priorities for damage.
Honestly, I see no reason a mage shouldn't be in the vision groups. Though to be fair, our group actually handles adds well so I get to spend about 98% of my time tunneling the boss.