Even if you don't plan to use any offensive magic, it's probably a good idea to get at least 10/16 faith and/or 10 magic to get access to some useful buff miracles/spells which don't depend on pumping the magic stat to do damage.
Evacuate = to return to the nexus
Second Chance (16 faith+15 Int) = Survive a mortal blow and refills half your health once per cast.
Anti-magic field (16 faith) = Prevents non-bosses from casting magic, VERY handy for the tower of Latria.
Cloak = Enemies have to get very close to see you
Cursed weapon(18 int) = Buffs your non-magic damage weapon for +50% more damage at a cost to health
Warding = greatly reduces physical damage taken
Water Veil = Reduce fire damage
Poison/death cloud = Useful against tough enemies by reducing their ability to heal while slowly sapping their health.
At the last two playthroughs (NG++/+++) on my way to the platinum, I played around with magic and miracles with dex build (mainly melee/bow use). These (for me) seemed to be the most useful:
warding (against the one shot non-boss enemies and main bosses when casting time wasn't a factor)
poison cloud (cheap boss kills)
water veil (for blue dragon final fight)
evacuate (when farming stuff)
soul sucker (farming BP skeletons/pure bladestones)
The rest of the abilities I used was from equipping special weapons or rings. So there wasn't much use in using magic/miracles at that point.