The issue for passengers departing St Pancras is do they use the high level platforms for the half hourly non stop service or the underground platforms for the 4 trains an hour semi fast service. In many cases the semi fast option will still be quicker as there will be less platform waiting
Same kind of Dilemma going south at Victoria where you can get the Southern or the Gatwick express.
My guess is airport passengers will naturally gravitate to the 'express' service as they do at Gatwick, without realising the same TOC operates a far cheaper 2 stop service, its just painted green and has a different platform.