I think it is potentially a good mechanic, but at the same time it feels like it probably needs very heavy watching/balancing. I like the idea mentioned earlier, of gaining free memory slots every few levels.
Really weak, or low level spells, I think should ideally be relegated to a 0 memory tier with its own separate number of slots.
The stronger abilities that take multiple slots, I have yet to try them out, but my fear with them is that their cooldown+damage/effect ratio might not be enough to justify taking up a spot for two other abilities.
