Let's take the salesman example. A salesman isn't literally making $150/hour. As a hypothetical the value might be $0 in hour 0, $450 in hour 1 and $0 in hour 2 for an average of $150/hour. Much of sales work is this way, their is preparation time, and sales time. There may be prospecting, etc. So even though they may not make the money during that hour, if you are burning their motivation to prep or prospect you are burning their ability to get through hour 0 and into hour 1.

Part of the reason to use an average is that it gives you a very simple, but generally applicable metric. There are always exceptions and the rate may change frequently or slowly. Those are nitpicks though. The general point is that your time should be valued against the best use of your time. For instance the hotel example only makes sense if the alternative to walking to the convention is working or more time networking to generate revenue. If the alternative is sitting in your hotel room longer watching TV then $160/hour is not the replacement rate.