Not every car is perfect for every situation and every task. Taking Microsoft's claim to this extreme, it would make sense. Each time you got into the car, there would be no need to re-adjust. The oil filter and all of the other parts would be the same, so there would no problem getting replacements. However, people are different.
Situations are different.
Therefore, you choose the best thing in each case. In once case it can be Linux. In others, it's Sun Solaris. The company I work for has all three and in use because no one has all of the features we need. However, if we need to configure the operating system beyond the basics, NT never comes into consideration.