Those of us who work in the usability and user experience industry know very well how hugely beneficial a usable system is when it comes to increasing a business’s income, reducing the time an employee requires to complete a task thereby saving money or increasing the enjoyment someone will gain from well designed spaces ranging from galleries & museums to the common reception area of a building.
Of course most people are not usability experts and are therefore quite reasonably sceptical about the level of benefit that can be achieved. This is why being able to calculate the return on investment is essential. It is always possible to look at the figures and guess what level of benefit has been gained but knowing, as a result of carefully agreed & collected data through the life cycle of the project, then performing simple analysis to prove the benefits is clearly a superior option.
A relatively small project may not benefit from such analysis, but any medium sized or larger project will likely benefit while the small additional cost of capturing and analysing the data should be a fraction of the project cost.