It does actually.
On this thread there are a lot of examples given by people that show that customization allows people to make the spells they want, a freedom that can never be got through a pre made spell set.
Ive seen a few I never thought of myself, such as a mark on target.
And it is doubtful that a pre made spell would let you do silly things like 1 fire damage over 300 sec + 10 light over 300 sec.
I agree with people that say that if I want to use such a spell, I should be able to make it.
Spell customization was the single thing I loved in TES over all the others.