You are right to say that public service employees are exceptional, and I believe Veterans Affairs Canada employees are as well. They work very hard, and they genuinely want to improve the lives of Canadians.
I do not know whether it is the department's decision to have temporary employees. I can only say that the budget gave it temporary funds. You cannot hire full-time employees without long-term funding.
It is indeed difficult to manage an organization when employees may leave their position...