LCCs only utilize "secondary" airports

Most industry commentators continue to emphasize that one reason low-cost carriers are so successful is that they only use secondary airports. They are accurate to some extent when they state that service to Oakland rather than San Francisco allows lower landing fees, less congestion, and sometimes, as an added bonus, quicker access to the city center. Dallas Love is closer than DFW, and so is London City than Heathrow. However, if all LCCs only utilize secondary airports, such as Oakland, Love, or London City, then what are LCCs doing at SFO or DFW? Why are they operating at this congested, over-priced primary airports? Did they have a malfunction and require assistance? Was it an unscheduled stop? No, the truth is that many LCCs operate to primary airports because sometimes the disadvantages outweigh what the customer wants...access to a destination. I have done an analysis of all the seats that LCCs flew in 2005 to all their scheduled airports, and it is far from the truth that only secondary airports are used.
A definition, and a word of caution, is in order. A secondary airport is usually an under-utilized, reliever airport that complements a city and its main, primary airport. Sometimes this airport is far away from a city center (Malmø compared to Copenhagen) and sometimes it is close (London City compared to Heathrow). There is no formal definition of a secondary/primary airport. A secondary airport is usually within a specific radius of a city center, say 50 miles or so, and is not the primary airport for that area. Here in lies the challenge: when doing an analysis what is a secondary and primary airport? What about an airport like Billund in Denmark? It is nearly 300 km from Copenhagen or Hamburg. It is not exactly in a tourist mecca. Is this airport a primary or secondary airport? I have classified it as a primary airport since BLL does not provide secondary, relief service for another airport nearby. So, Ryanair's service to BLL is noted as service to a primary airport. This was done for all the destinations that Ryanair served in 2005. The sum of the annual ASK to secondary and primary airports was divided by the total annual ASK to get a percentage, which is what the graph is showing.

Maybe not surprisingly is that Ryanair, easyJet, and Southwest top the list with the highest percentage operated to secondary airports. The caveat is that it is not 100%. Southwest is only operating 35% of their annual ASKs to secondary airports. All that service to Saint Luis, Pheonix, Salt Lake City, San Diego, LAX, and Las Vegas is service to primary airports. Now, please let me know if I have listed an airport wrongly. My geographic knowledge of the entire globe is limited. No airports served by Virgin Blue or WestJet were listed as secondary. Maybe this is error on my part and I apologize. Please let me know and I will adjust the figures. I had to use an online map service and compare airline operations to airports nearby. If it seemed reasonable that some passengers would substitute airports I listed it as secondary. As with anything it is very subjective. Of course, some LCCs helped me out by marketing a destination as serving a not-too-distant city.