Say I'm playing a mage (in DFRPG or Magic 4e systems) and I have a "casting item" -- that is, an enchanted item that lets the user cast a particular spell but requires the user to provide the mana (meaning it doesn't incorporate Power as one of the enchantments, or not enough Power to maintain the spell and be "always on").
I also know the same spell (independent of the item) at skill level 15, which would be the minimum level the casting item could have (because the enchanter must know Enchant and the spell to be incorporated at that level or higher in order to perform the enchantment, and the casting level from the item is the lower of those two skills by the enchanter or lowest in the Circle of enchanters).
If I cast the spell by my own skill, I get a 1 FP discount at casting and each time I maintain the spell, but it counts as a "spell on" and reduces my casting skill for any other spells I cast while it's running. Alternatively, if I cast the spell from the item, I don't (apparently, per M19) get a discount on the FP cost. Since I'm still having to maintain the spell each time it expires, do I still have the "footprint" against my effective skill to cast other spells? If so, what (if anything) would be the advantage of casting from the item vs. casting from my own memory?