Should it be legal, in an absolute normative sense? Hell no. Is there any workable way to ban it in the remotely foreseeable future? Also hell no.
In the abstract sense no, pornography is much more harmful IMO than most mind altering substances. Since we're dealing with real life, I'm not really sure how to go about it. The stuff's poison for the mind, but it's virtually impossible for the police to do anything about it. The best we could do is ban some of the worst bits of it and go after it like we do kiddy porn.