We have Leopard and we love it. Macs are so fun and easy to use. Why buy anything else?
That's easy to answer. There are a ton of specialized software packages that simply do not run on Apple machines. It's impossible to justify the cost of developing specialized software to operate on Apple computers when Apple's market share is less than ten per cent.
Apple is certainly idiot-proof but a lot of people expect more than that and look at their computers as an important tool to get a complex job done.