The site you linked to says quite clearly that you should register to get:

""" # a legal presumption of the registrant's ownership of the mark and the registrant's exclusive right to use the mark nationwide on or in connection with the goods and/or services listed in the registration; # the ability to bring an action concerning the mark in federal court; """

So in other words, if you don't register, you don't have these things. Yes, you have a trademark, sure... you just can't do anything about it.

"The owner of a federal trademark registration may sue in federal court more easily. It is worth noting that even without a federal trademark registration you still may be able to sue in federal court, but it may be more costly when proving jurisdiction (i.e. diversity of citizenship & amount in controversy over $75,000). Even if you can't sue in federal court, you can always sue in state court. However, attorneys usually prefer federal court in trademark cases for perceived procedural advantages."