(this should probably go in the Rules Forum)First of all, Magic Missile can't be used, IMO, to charge a Rod of Absorption, since it is not a single-target spell.
Also, when the rod is in hand, the wielder cannot choose not to use it. If the wielder receives a cure spell, the spell is absorbed.
For spellcasters that prepare spells, the spell must still be in the caster's "memory" for him to use the Rod to cast it. If your wizard has used all of his high-level spells, a Rod of Absorption won't do him any good. For spontaneous casters, there are no problems.
Finally, the way I read it, the Rod can't be used over and over again. Once 50 spell levels have been absorbed, and 50 spell levels have been used to cast spell, the Rod is burnt and useless. The Rod doesn't "lose" spell levels when the spells are cast.
50,000 gp would be very cheap for a magic item that basically lets you cast 5 9th-level spells every day for free, since 5 9th-level scrolls would cost ~20,000 gp. Every day.
Hope this helps!
AR
ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHyuxYywpp6rXay2tbSMq6adZZ%2BbeqKu0qipqayZpLturcGuqp5mYWZ%2Fd3yTaA%3D%3D