Luton will end up with 2 express, 4 semi fast and 2 all stopper services per hour, total 8 trains per hour.
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.
As stated the airport and council were lobbying for 4 express trains per hour so got half of what they wanted which will be a disappointment for them. Also unless things change with a change of franchise, group saver tickets are not accepted on non stop services to and from St Pancras.