Even if they are not used for landing, main thing is that they use same fuel tanks as those used by Draco thrusters for orbital propulsion and deorbit, so no weight is wasted (weight of the Super Draco engines themselves is not that big). Overall payload loss for the abort system is about same as if using the escape tower (ca. 800kg if i do my math right), but escape burn is possible throughout the ascent, not just in the first 3 minutes, and there is no additional safety risk from it (if escape tower failed to separate, astronauts will be doomed, here there is no such risk).