My clients understand that, but they also expect me to mitigate the risk that sh*t will affect them. In many cases, arriving late would render the trip pointless, and require a rescheduling to a different day/week/month altogether. Obviously, if a snowstorm in Frankfurt or a flood in the Netherlands closes the airport for multiple days on end, well, that's a minimal enough risk that I am willing to take it. A less minimal risk is that my flight into FRA is delayed 2 - 3 hours, resulting in missing my onward connection, which is a once a day flight. I prefer enough slack that even if I get the next flight a day later, I will get there late but not at the point where my trip no longer has meaning.

It really depends on the kind of business you are in and what kind of potential delays you are talking about.