By not being ready at the holding point they loose opportunities to get airborne sometimes costing them 10 minutes or more holding.
At LHR we advise ATC when requesting our PDC or on first contact with delivery that we intend to taxi out single engine. A little reminder to ground when calling for taxi usually means that when we go over and monitor tower the controller will 9 times out of 10 advise us of how many minutes we can expect until it's our turn. Most guys are cautious and will actually have No2 up and running earlier than this advisory time.