Even today, if you're ever forced to fly Spirit, buying them at the airport is the only way to avoid extra fees. (Odd, considering how it's the exact opposite on every other airline.)

The fee to book anywhere else - including their own website - is a minimum of $18 per person per direction, so if you have 3-4 people flying, it might actually be worth your time to do it. We don't fly them often, but they're really the only practical way for our non-Delta-elite family to get to Detroit. Buying at the airport avoids $144 in junk fees per trip. Fortunately I've been able to do it when I was already at the airport for some other trip...just had to find the Spirit counter and wait in a 5-10 minute queue.

Other than that, I bought United tickets at the airport as late as 2009. Certain types of vouchers pretty much required it up until that point.