". Your successful melee touch attack applies a 1d6 penalty to the target’s Intelligence, Wisdom, and Charisma scores."
Seems to imply you roll 1D6 and apply the damage to all three stats, otherwise I might have expected wording such as
"1d6 penalty to Intelligence, 1d6 penalty to wisdom and a 1d8 penalty to charisma."
Bonus spells/day would definately be lost (In a similar way to losing spells in spell slots lost to level drain). Further :
"To learn, prepare, or cast a spell, the wizard must have an Intelligence score equal to at least 10 + the spell level. The Difficulty Class for a saving throw against a wizard’s spell is 10 + the spell level + the wizard’s Intelligence modifier."
I would highlight the word learn. Maybe Im mean but I would rule that if your stat drops to the point you cant learn that spell you lose it. In effect you no longer comprehend the magic and it slips from your mind. I would only apply this to prepared spells, ie cleric,druid,sorcerer,paladin,ranger. In effect I would rule that on restoration of the stats you get back slots but they are "empty", prepared spells are lost, but the slot is still there for spontaineous casters.As some sort of justification I vaguely remeber a reference to preparing spells actually being the process of casting maybe 70-90% of a spell and leaving just that last few mostions unfinished, with a reduced stat you lose the ability to retain these quasi spells.
The spell isnt clear but I would treat the stat loss like a level drain, and work out lost spells accordingly (ie random loss of spells from slots lost). But Im mean and I like the idea of inflicting 2/3 of thse on a cast to drop their tops spells and DCs

To be fair 1 touch of idiocy (unless you roll a 5or a 6) is unlikely to do much, most casters by 3rd/4th started with 18 or 20 in int (maybe 22 with SCP) and maybe have that up to a 20-22 with an item. Unless you use the spell repeatedly it will have limited effect.