Heavy traffic. You've just deiced. It's snowing and the clock is ticking, there is a lot of inbound/outbound traffic, the airport is closing soon and there is an unexpected runway-change. Whereas you had a long taxi ahead of you before, you now find yourself 200 metres from the new hold with a string of landing-lights on final, albeit with enough time/space ahead of them for you to make it out if you are quick enough to get your ducks re-arranged into the new row. 'Ready upon reaching' says I, in the hope of informing the controller that we CAN make it, should he be able to work us in.
Unacceptable?