I did tower for over 30 years and never had any problems with conditionals, mind you it was better phrased in those days ie 'after the departing' or 'behind the landing'; perhaps a trial with this 'old fashioned' phraseology should take place. NB I would never give a conditional to a vehicle; there's no way of knowing how competent the driver is and when we had miltary aircrew, they asked us not to do it anyway. Also as someone else said, if you are doing air and GMC combined plus vehicles, you may need to take advance actions using conditionals in order to expedite traffic; I get the impression that the people who decide on these trials don't take RTF loading onto account.