I also put only 5 or 6 points in Spirit so far, but all the other points to reach 15 are from boosts on equipment & Spirit Charms I bought and put on gear
There is also another "pro" when investing in a warrior that concentrate on Magic Damage Melee Weapons :
now you can choose to make some great (and fun) strategy steps like for example :
the battle start and you jump nearby a group enemies or use Rush Attack to fast end up in the middle of a group ... do a Wirlwind attack and use Firewall ... and because of the higher Intelligence all the magic spells are even deadlier !
Okay, I know in that way you are not a pure warrior, but why would you use only warrior skills while this fantastic RPG
give you a open skill tree ?