The problem with normativity is that "the ideal is always threatening to collapse into the real" (Pierre Schlag). Essentially, the danger is that what we should do, will simply become that which we already do. At that point there is nowhere else to go. Critical reflection ceases. Normative evaluation ceases to be fruitful because all it does is tell us to do what we’re already doing. What’s the point of that?
For this reason, a critical stance is preferable to a normative stance.