Why not? I used easyJet for all of our HackFests. Where do you see a problem?
a) Such airlines do not offer reliable service, in comparison to a train: You can be shure that at least SOME of the Hackfest participants will be faced that they "can´t take the cheap flight as expected", due to
- missing capacity ( no seats )
- does not fly at the needed days
- does not fly from next to the living place, does not fly next to the conference location
b)
E.g. London: The travel by train from the "London" airport to REAL London is more expensive than the flight from a German airport to "London"
c)
Cheap airplanes even don´t fly from many major (German) airports, but from minor ( German ) airports, so you have to travel to exotic locations sometimes even without railway station, or the costs with railway to come to the exotic place are even highter than the flight costs.
With Hannover as hometown, I even have luck that SOME cheap flights start here, BUT
The cheap airport for Frankfurt ist NOT Frankfurt but Hanau.
The cheap airport for Hamburg is Lübeck.
The cheap airport for Bielefeld is Paderborn.
The cheap airport for Berlin is NOT in Berlin, but a former military airport in Brandenburg...
and so on..
Same with targets: if the REAL targets of the airplanes are NOT next to conference location, you have much extra traveling costs...
d)
In 2008 I wanted to travel to Graz, Austria ( second-biggest town in Austria, behind Vienna )
http://en.wikipedia.org/wiki/Graz.
It was impossible to take a flight, especially no cheap flight,
as (at that time) Graz was not target of cheap flights ( I am not even shure if they have an airport at all ), especially not of direct conections from my hometown Hannover.
And "taking train plus flight" does not work either,
as planes don´t wait for trains, and vice versa,
with fixed booking of trains and planes you come easily in a situation where you can´t continue traveling with your paid tickets, as you did not reach the only ( paid ) connection...
on my last railway journey to Stuttgart last autumn, I met a guy with a "cheap" plane ticket, but due to a 2-hours train delay, probably he missed his flight and so a 800 EUR value of hotel + flight tickets...
e)
Cheap railway tickets don´t become much more expensive if you double the distance, within Germany :-).
So flight + train is usually more expensive than "just train".
Sincerely
Rolf