Am I the only one who remembers the cannon talking about how focused practitioners are actually not capable of being wizards? How they aren't just not as learned, but actually lacking the strength to be able to do more? Thematically they aren't super awesome with the one thing they choose to work with, but actually barely capable of doing the one thing that they have the affinity for.
Or am I just crazy?
Some Focused Practitioners lack significant magical oomph to pull off much. Others have the potential for more but simply haven't grown into it. Still others are Wizard-level within their specific niche, but don't have any talent at all outside of that niche (as in, even less than Harry's talent for veils, in
everything). Mortimer, for example, is hinted by Leanansidhe to have the magical "batteries" to pull off some major stuff, but he lacks the right talents to even become a sorcerer. That's part of he was targeted in Ghost Story.