I think parents have to be discerning. Some children and teens are spiritually unstable, and reading Harry Potter books may lead them into investigating ritual magic seriously. (Someone pointed out on another thread that bookstores are marketing books on witchcraft and even Satanism in conjunction with the H.P. blitz. I noticed some of this myself when the new editions of LOTR were issued, in conjunction with the films.) In my opinion, a young person in this situation should be distracted from any fantasy or magic-related literature, not just Harry Potter.
For me, the point where any fiction becomes a spiritual danger is when the reader wants it to be non-fiction.
I think C. S. Lewis says something like that in his essay on Fairy Tales. For most people, fairy stories are less dangerous than stories about becoming rich and famous, because they don't feed into the vein of self-absorbed wish-fullfillment (Lewis was writing at a time when Freud's take on literature was dominant). But I agree, if a kid seems to be taking the idea of magic seriously, then he needs to be educated or diverted to something else.
The difference between magic and religion, which some folks on this thread don't seem to get, is that religion is about service to God, and magic is about power over others. It's clear to me that Rowling disapproves of greed for magical power in her stories, and approves of magic used wisely as a trust in the service of others, but perhaps naive readers might not get the distinction.
My ten year old agrees. I let my children be exposed to culture while taking on the added burden of talking about what the bible says (ie in this case OT bans on witchcraft, and NT advice to not make your neighbor stumble ). In the long run I see this as the best way to protect my children. If they can discern evil and be equiped to consult the bible and use it as direction for their lives, I have accomplished more than out right bans.
While discussing Harry Potter, I asked my youngest son of ten, whether he thought Potter stories could tempt him toward occult activities. His answer was a quick no, followed by a concern that it could affect other children his age. The danger here is not that everyone who reads Potter will become a satanist. The danger is that some will and that Potter will be one link of the chain that leads them there.
Well reasoned. Potter books are not universally bad for kids, but in the wrong hands, heck, manure can be deadly!