It's not paranoid at all. It's perfectly sensible. In addition to techno nasties mentioned earlier, your website may be open to people posting adverts, porn, links to undesirable material etc. At one level, this is a pain. At another level, the material may be illegal.
It all very much depends on what you are trying to accept and the scale of your problem. If you are running a small newsgroup, you can isolate content before letting it go or provide a mechanism for users to report content that they don't like. You can then operate an auto-exclusion rule when three people object to the content. If you are setting up the next version of Flickr then you will need to add some technology into the mix.
You can automate the approval of most text content if you are able to compare it to examine it. The problem is that humans are endlessly inventive when it comes to spellings of Ciali5 etc.