Unlike most players( not referring to you, Six [img]tongue.gif[/img] ), my spellcasters tend to have a wide variety of spells. I also have a standard set of opening spells to cast( usually level 7 and above).
Now... it is true that I can just pause and clickity-click-click all the way to the right of my spellbook to cast those spells... but it gets
tedious after a while.
My opening spells are standard, so having them in the Quick Spell slots saves me time and preserves my sanity from having to scroll through my spellbook over and over again.
For spellcaster fans like me, who mainly use spellcasters all the time, the Quick Spell slots are truly divine. More so since I play BG2 with the AI turned off, so I have to cast each spell individually. You may come to appreciate these slots more when you play with a party that contains 2 Mages, 1 Cleric/Mage, and 1 Druid.
I prefer to spend more time casting my spells than looking for my spells.