8 ranks is a +24 bonus
This is only true for RMFRP/RMSS. In RMC it would be 8 ranks = 40 bonus.
Personally I would never allow items to have their own directed spell skill unless they were imbued with intelligence and even then never let them have more ranks than the maximum possible for that level of intelligence. (This may be different for RMFRP)
Using the skill of the item raises other problems as well:
1) Which level bonus does the item have? (magician if the spell is from a magician base, astrologer if taken from an astrologer base?).
2) What is the agility stat of the item?
3) If it is an item attacking with RR spells, what is the BAR of the item? (magician if the spell is from a magician base, astrologer if taken from an astrologer base?).
4) If using CC, What is the style of the item? (Based on the imbedders abilities, based on the caster of the spells ability, dependent on intelligence?)
5) What about daily scrolls? Do they use the directed ranks of the scrolls level or do you use your own skill when using daily items, if it is a scroll?
When answering all of the above questions I would look to the intelligence of the item and if it had none I would use the skills of the wielder. I would probably allow the users skill to be applied if it was greater even if the item had intelligence unless there is a conflict of interest (Will) between the user and the item.