[Bruno]

I'd consider this not on a rules and world-design basis, but as a result of player dynamics.

EG, if only one player is willing to play a caster, but the group is beginning to argue about how they need more than one caster type and try to pressure another player into doing something they don't want to.

Or EG if I only have two or three players, the players are anxious about the lack of variety, and we've decided not to go the multiple-PC or NPC sidekick/hireling route.

I'm perfectly happy to run a game with no spell casters, or all clerics, or any other party composition my players come up with. It's based on what's sucking the fun out for my players that I'd patch this on.