Although slot adherence should be insured normally there are occasions where it really doesn´t help but in fact worsens the result. In our center we´ve got quite a few routings that involve very short (repositioning) flights of 15-20 minutes. Sometimes there´s only one or two sectors involved in such flights, but still the TWR guys won´t clear a departure for take-off unless the CFMU has exempted the flight from the regulation. This takes a while of course (and sometimes serious discussions too) preventing us from using an opportunity the "real" traffic picture provides. This often happens during regulations due to WX.