I must admit that I really enjoy a Vancian styled magic system in my D&D. Maybe it's because I'm older and have fond memories to AD&D and 2nd Edition. Maybe it's because of the preparedness I have to consider when I play a spell casters (and the roll playing options that follow, "What do you mean you did choose fireball today?"). Maybe it is because as a DM I like PCs clever use of spells for unique situations (my favorite was a PC Wizard using "Gaseous Form" to get past a very nasty trap). Maybe it's because every potential NPC spellcaster the PCs face could provide a new source of spells. As a DM the spells I can drop into a campaign might give the PCs an idea about what threats may be in the future (Ray of Frost? Oh maybe that's for the Hellhound we found guarding the elemental forge?).