Economists argue that slavery is inefficient in all but a few instances for the reasons cited above, but also for three others: slaves have no motivation to work other than to avoid punishment and so are likely to do as little as they can get away with (not to mention, as many American slaveholders complained, employing techniques of passive resistance such as 'accidentally' breaking tools and machinery); slaves can be punished, but not fired, so slaveholders are basically stuck with their workforce, however incompetent it may be; and slaves are an expense all year round, not just when work is required to be done.
The economic advantage of slavery is only apparent in situations where free labor is unavailable: that is, in places and for jobs that no-one would willingly do, but slaves can be forced to undertake, such as the Caribbean sugar industry, where conditions were so harsh life expectancy was measured in months.
Orange is right, Amarr slavery is explicitly described as including educated and trained slaves who work, for example, in civil service jobs, but the inefficiency arguments above still apply to educated 'professional class' slaves. For example, if you have a slave doctor to tend to the medical needs of your family, you have to feed, clothe and house her all year round, not just pay her when you're sick; you're stuck with her medical care (certainly without paying again for another doctor); and your slave is not motivated to provide you with the best possible care to ensure return custom.