The main problem I found playing a wizard in Deadfire, wasn't the number of spells taken during the level up, but how the spells are spread out of multiple grimoires and how the majority of these grimoires doesn't have synergies between the selected spells.

This bring the player to chose spells to fill the gaps of their grimoires, but as you find another grimoire the problem still rises. You can have your preferred grimoire and build up your spells around them, it's what I do, but sometime you want to build a more flexible caster without the necessity to focus only on one grimoire.

The grimoire should support the spells the wizard has learnt, not always being the catalyst.
You can build a wizard both the ways, but at the current state the incentive is to build around the grimoire.
More quick slots for wizard is an incentive to bring more grimoires in combat and being less focused to only 1 or 2 of them.

That's why of the "Knapsack" talent, the incentive to care less about grimoire because you have access to more of them. 
Why so early? Because I see the talent like an alternative to "Arms Bearer" but for the caster, they compete the same spot.

How many times you started a fight, or get ambushed while having on your hand the last grimoire used, and, it's not what you usually use to start a fight, or just you don't need it?
Spells usually have long cast time, so when you switch a grimoire you need to consider the time needed to switch, plus the time needed to cast.

Having a "Quick Switch" version but for grimoire is meant to address these two issues.
The fighter's talent reduces the cooldown of -1.5s, I find it too much, "Bookworm" does for -1.0s.
The recovery time is 2 seconds. Having a 1 second of recovery time seems fair.
Like for "Knapsack", also "Bookworm" is the caster version of "Quick Switch", it competes for the same spot, and because of that can be unlocked at power level 4, like "Quick Switch".