TwinAisle makes some good points, but a business website must be in tune with potential customers.
Even Ryanair is making apologies for its website ATM, stating "We regret, due to system slow down issues, we have been unable to display the tax inclusive fare box on this page, since 25/06/2008."
PCSs make an inordinately high number of hits on the reservation system - they are continuously interrogating the airline's systems to price flights, the overwhelming majority of which do not get booked. This makes the systems slower for everyone else using the site, which pushes up the "bomb out" stats - ie, the number of customers who start a booking and then get bored and go somewhere else because the site is slow.
How do Amazon cope? Their site actually uses customer feedback to dynamically create the pages on the fly and I've yet to see it crash.