"If you try to create a perfect design you will expend a huge amount of effort. You will go through all sorts of behavior patterns. You and the architects meet and think about this perfect design mightily. And you argue for hours over, say, whether next should be synchronized. It is a vast sink of time. The difference in effort between delivering something perfect, as opposed to something good, is significant. It just isn't worth the trouble in 98 percent of the cases," says Ken Arnold in this interview:

http://www.artima.com/intv/perfect.html