Peter Merholz has a great post about the cumulative impact of complexity:

“Objects aren’t simple any more. They don’t just turn or push. They behave. And these behaviors are often played out over many steps, in particular orders. And each step is an opportunity for failure. Through the work that my colleagues conducted on business value and user experience, I learned the six sigma concept of ‘rolled throughput yield.’…

Basically, this means that the more steps you take, the likelier failure is. Even each step has a high probability of success, when you add them up, the likelihood someone can get through all of it becomes startlingly low.”

Another way of thinking about this (from more of a marketing and sales mindset) is “the more people you lose at any given step in your sales funnel, the worse your overall results”.