If it's like desktop anti-virus, it will have its own vulnerabilities, take up more resources than I'd like, cause buggy behaviour or incompatibilities with other apps, and feed me false positives too often.

I don't need that on my phone. Since the only real malware we've seen for the iphone involves jailbreaking and then not properly managing your phone, I can do without.

I love watermarks that can be defeated with a spellchecker and a thesaurus!