Debating online is too easy. We can't see one another's faces, and we normally have absolutely no idea who the other people are. Online debate often goes from being conversational to adversarial in seconds, and can result in name-calling quite quickly. Debate rarely causes another person to change their mind, and is especially unlikely to do so online. However, it can also be done in a moderated way, such as is being done here, to reduce the likelihood of all of these potential problems. In that way, it can be educational and even fun.
Online debates are often done under a mask of anonymity and are often not about communicating ideas and thoughts but more about "pwnage" (internet vernacular for one upping an opponent) and impressing online friends than they are about an actual debate as we understand the term. They're no substitute for real discussion.