What can't guarded fragment of FO express?
You're correct that, if the structure you're working with has a universal relation, then using that relation as a guard is equivalent to having no guard at all. However, you don't get to pick the interpretation of the relation symbols. For example, if you're trying to write a formula that is satisfied by exactly the class of 3-colourable graphs, then you should think in terms of an adversary supplying you with an arbitrary graph and your formula has to give the right answer, without being able to rely on any particular properties of the structure you're given.