I think Southwest will fulfil its passengers needs a bit more often than 9 times out of 10. Closer to 95 times out of 100. The problem is that when they fail their passengers, they fail their passengers much more miserably than other airlines.
One correction: All major airlines have fares that incur no charges to make changes. It is only heavily-discounted fares that they charge penalties for. Southwest's fares are very low to start with, so they don't offer heavily-discounted penalty fares.