OK, my observations of Wicca;
Wicca is very spiritual. It seems to attract the agnostic, and not so much orthodox atheists. I have seen rituals where a prediction or wish for the future is writen in impromptu symbology on paper, then burned at a ceremony to make it come true. Sounds pretty rediculous to me. Not much different than the catholic communion ritual. Here ritual seems to be the operative word. It seems that if there is ritual attached, it somehow legitimizes radical behaviors. In the case of Wiccans, that behavior appears to be running around the woods naked. OK, nothing really wrong with running around the woods naked if that's what you want to do, but if you truly believe that this behavior has some impact on nature or the world around you, it's my guess that you have a circuitry problem in your cerebral cortex.
I say we all run around the woods naked because it seems like a good idea. Makes more sense to me, but I doubt you'll ever see me do it.
To answer your question though, Wicca isn't "harmful" unless like other "cults" it is pressed on children.
I don't know that religions per se are harmful as much as the people who "practice" them. I could care less what a person believes in terms of god, or afterlife, but I care very much about how they act on their religious beliefs. Proselytism is evil in my book, because they come and disturb people. Megachurches with megamillionaire pastors bother me, because that money would be far better used serving the poor and the sick, and victims of natural disasters. If a Christian sacrifices in order to help others WITHOUT hinging it on converting to Christianity, then that is a good thing, and I admire them for that. But when missionaries go out ostensibly to help 3rd world countries, but with the real objective of converting them to Christianity, I find that revolting. They just found an untouched tribe of about 200 people in the Amazon, and while the Brazilian government wants them left alone, what do you bet that some missionary is going to sneak in with his book and his diseases and wipe that tribe off the face of the earth?
The only religions I respect are those which keep to themselves and leave everyone else alone. I DO think it is possible to coexist with religious people, and there ARE sincerely good religious people, but the crazy ones are louder and more obnoxious. I was raised in the reform Jewish tradition (although my family is now all atheist/agnostic), and was very carefully taught not to bother the Christians. I just wish they and the Muslims would return the favor.
Religion can be harmful when it causes people to make irrational decisions. The major religions are harmful, especially in a democracy, when they affect us by their power of numbers-- changing laws that oppress minorities like Atheists and discourage scientific research like stem cell research and other fields that could prove beneficial.
I think Wicca is not harmful now because there are few adherents and they are generally peaceful. What would it look like if Wiccians were in the majority? I'm not sure. If Wiccians were in the majority would they be opposed to reason? Would we have "non-witch" hunts? Would they insist on having their rituals performed in public? Would they favor politicians that at least give lip service to their beliefs?
Is it useful to replace one set of irrational beliefs like Christianity or Islam with another like Wicca? What's the difference? Our society progresses when we are rational and free thought and science is allowed to flourish. What religion would be the best custodian of our freedoms? I think the answer to that question is "None". We are far better off as a secular society that promotes free thought and freedom from religion.
If Wiccans were the majority...well I have met some pagans who talked about killing all the Christians "why not, they've been doing that to us for eons". I can't stand the mentality of killing people now because of something that people did hundreds of years ago. Nobody can change the past. I've also read interviews with pagan fundamentalists who encouraged all other religions to leave the planet because "pagans are the only ones who love the earth".
There are others, probably the majority, who are more tolerant, of course. But if they were the majority I think it would be the same church/state struggles. Pagans I've known basically have thought pagan prayers are not a violation of church and state, or the "it's different because my religion is benevolent" attitude.