It isn't a matter of ability points, it's a matter of attribute points.
Yes? So?
You'd need enough Dexterity for the bow, and enough Intelligence for the spells... and not much left over for Constitution, Speed, or Perception.
Not really true. You will have less in any specific attribute but you will have enough overall.
Plus, there is enough equipment and items that boost various attributes so you can make up the difference more or less easily.
Ive been playing jack of all trades since the start, and went through the beta with just two characters, without Lone wolf and glass cannon talents which are just cheats.
It all works fine.
Pure "class" or rather, a focused build will have its specific skill and attribute higher then the jack but jack will be more diverse. If im taking Madora i keep her focused most of the time, just so i can see the differences. Which isnt that big. But when i gave her one level of magic skill she became a killing machine.
With additional feature of being able to cherry pick and change the spells someone has learned.
It only comes down to what any player wants to do or prefers, personally.
For me, Jacks are much more fun and interesting, for someone else a focused build is better.