When it comes to selecting a roofing underlayment, homeowners are mainly concerned with the tradeoffs between performance and cost. An underlayment is not going to be visible so aesthetics plays no role in selecting an underlayment.
Some homeowners are more cost conscious than others. One homeowner may put quality and durability above all other factors while another may be looking for the least expensive solution. Not all homeowners may be in the market for a 30-year roof life. Climate may be a factor, too.
For the contractor, the most important considerations are suitability for the application, walkability, exposure limits, ease of application, and costs. These factors are obviously interrelated. For example, any of the first four factors can influence cost; similarly, improving the exposure limits may affect the ease of application; and so on.
Different contractors may stress different factors. For some, labor may be a more important consideration than materials costs. For others, exposure limits may be the overriding factor.
Furthermore, ease-of-application affects labor costs, which in turn affect total installed cost. Labor costs could be reduced using a self-adhering underlayment; yet such may be heavier to lift to the rooftop than other options. Ultimately t is up to the individual contractor to weigh these factors and select the most-cost-effective solution for any given project.