Slander and insults are also allowed as long as they contain any kind of constructive opinion, info or something else substantial and aren't off-topic. For example, posting something like "you are dumb" will be deleted as it contains no meaningful content. However, if the post is somehwere along the lines of "You are dumb. This is wrong because this website/thread/etc. has explained it's not right", it's in most cases accepted.
Do you really mean slander? There could be legal implications, certainly there are slander (and libel for print and other media) laws here in UK, and I'm sure variations of the same in other countries. Completely agree (FWIW) that if a potentially contentious statement is made it must be backed up with evidence.
Cheers, Ngaio
Edited the post to change it to a bit more mild meaning.