Employee abuse is not good business. It results in high turnover, degradation of work quality, hidden sabotage and diminished innovation and creative contribution. It is good business to relieve excessive temporary work pressures with outside help. Job requisitions can't be filled in a timely fashion. When requisitions go unfilled for excessive periods of time it means that work deemed important is not getting done.
Workloads are sporadic or seasonal.
There are jobs that are not full-time by definition. Rather than shuffle and disrupt a work force to accomplish tasks that are sporadic, hiring an independent worker could be a better option. Growth occurs in unpredictable spurts. Unlike the regular ebb and flow of work that can be planned, these are situations, like greater than expected success for a new product, where work comes in sudden spurts. Fresh thinking, new ideas are needed.