To use them consistently, yes.
If you don't have 8 INT (at least for some spells), your spell will fail sometimes.
I believe each level under the required level removes 10% hit (for buffs and debuffs) and 10% damage (for the damage component).
More intelligence buff by 5% for each supplemental INT, but that's oftentimes useless for pure buffs (you only need 100%).
