Thematicly focus item's are a big deal in the Books and everyone not using some would be considered strange...
So far, only one player in my campaign has taken advantage of the house rule, and what they did was trade in one Thaumaturgy focus item slot to pick up an extra +1 specialization in Entropomancy: Control +1 while using the remaining Focus Item slot for a mojo bag foci providing Psychomancy: Control +1. That is the Freelance Houngan who chose to do so. The other members of the party (a Hedge Wizard, an Ectomancer, and an Apprentice) all have either kept their slots as focus items, or exchanged some of those slots for Enchanted Items or Potions Slots.
The major point of the house rule is to provide an option for a character who thematically would not use Focus or Enchanted Item/Potion slots very much, and not as a way for players to munchkin build their characters.
And I agree that most casters would likely have at least a couple of items which they would use. However the novels and short stories have given several examples of practioners who don't have any apparent or at least obvious, focus, enchanted or potion items.
Cowl, Grevane, Corpsetaker and the spellcasting Denarians all come to mind as those who don't necessarily use such items.
The other important part about the house rule is that the specialization pyramid must still be followed, and I don't allow players to start out with a specialization bonus above +1, since the campaign's starting refresh is only Feet in the Water level.
-Cheers