The problem with hourly pay is that people consciously waste time. It can reward those who take longer to complete a task. A salary provides incentive to complete your work quickly. But yes, supervisors should do what they can to make everyone's work/salary balance proportionate.
My company, and many others, measure the effectivity of salaried employees based on the number of projects and time spent on these projects in which an individual has been involved. The fucked up thing, though, is that it is often the role of senior staff in an office to bring in new projects, and when they fail to do so, their employees' effectivity suffers. When layoffs are made, it's almost never the senior staff who are let go- it's the mid-to-low-level employees.
"LIVE OR DIE, MAN??"