Keep in mind, too, that there are different ways of defining "powerful" when you're talking about spellcasting - there's "does one thing extremely well" powerful, there's "can level buildings with fire blasts" powerful, there's "bargains with demons for power" powerful, there's Ebenezer "What A Big Stick You Carry" McCoy, etc etc.
I imagine that what usually happens is that because the Venatori are allied with the White Council, anyone who shows significant enough magical ability gets referred to the Council and trained there. As an organization, the Venatori probably aren't equipped to deal with the potential dangers presented by having an individual that powerful in their ranks. However, I'm often fond of saying, especially about roleplaying game settings, that any info presented can automatically be read with a "as far as anyone knows" in front of it... the idea of the Venatori keeping their own cadre of wizards in secret is a compelling idea, especially if one of them starts getting ideas about what should be done with the organization's vast store of knowledge.
-Lenny
