Depends on how many abilities you consider absolute "must-have" on that character no matter the situation. There are plenty of variants of a "hybrid" character and they have different numbers of "essential tools".
Memory slots in most cases just provide you peace of mind, as it makes it less likely that you are thrown into a sudden fight without the right tools. In the worst case scenario, you can simply reload and re-memorize. Except for the toughest fights that drag on for hours, you won't ever get to use every single one of the abilities you memorize.
If you constantly find yourself wishing you had memorized this or that instead of that or this, or
If you can't stand the hassle of reloading and re-preparing your toolbox for incoming fights,
then get another 2-3 slots.
Last edited by Try2Handing; 26/09/19 05:08 PM. Reason: oops