With this whole main airports debate, is the crux of the matter not advertising a major city or destination and flying to a smaller airport miles away? Easyjet are advertising Doncaster as Doncaster, not as Leeds or some other conurbation somewhere. The main airport for Doncaster, is surprisingly Doncaster.
Ryanair on the otherhand (amongst others) advertise a major city or destination, but will not actually fly you to that city's "main" airport, good examples being Hahn, Beauvais and Girona / Reus.
My take was Easyjet fly, not necessarily to only main cities or airports, but will fly you to the destiantions main airport (if that makes any sense).