I've worked the Lucozade station at 23 miles a couple of times, and if we left the caps on the whole race would grind to a halt as tired disoriented people slowed down or stopped as they tried to get the lids off their drinks. They're hard enough to remove at it is without trying to do it when tired and on the run. Also if it's cold then it gets even harder to get the tops off - I remember handing out leaflets after the Reading Half a few years ago when it rained, hailed and sleeted on everybody running over 1hr 20min, and I ended up spending more time opening drinks and the like for runners whose hands were too cold to do it than I did giving out leaflets.