With regard to casual workers, the possibility you mention exists. If these people are used in the proper way, in other words for periods of up to 90 days within the year, then all the better. It is a way of responding to immediate needs. If, however, people take advantage of this opportunity to circumvent all the requirements pertaining to entry into the public service, including the merit principle, then there is a problem, one that we have already brought up.
Furthermore, you were inferring that certain departments resort to this process so as to not fill positions permanently. This would require taking a look at what is really going on. The act is quite clear with regard to duration. Casual workers cannot be employed for more than 90 days per year. I am perhaps not aware of the cases you are alluding to. However, when casual work is used appropriately and managed according to the law, then it can be very profitable for everyone.