Well, if the output doesn't conform to the format it's useless. If the model can't produce good and correct output then it's simply not up to the task.
In my experience, LLM responses result in a fair distribution of outputs that do have semantically useful outputs but do not precisely adhere to the requested format. If I chose to use a strongly typed language for LLM parsing, perhaps I would be tempted to eliminate complexity and simply throw structural outliers away, and explain to the suits that a certain percentage of our queries/expenses are unusable. Instead, more sophisticated coercion techniques could be applied instead to increase output utilization.
That really strongly depends on your task. Lots of tasks can accept a non-zero failure rate in return for better results on the successful cases. I'm not sure I can think of any off the top of my head where you'd use a LLM and can never deal with a failure, particularly if you're using an external service where you're guaranteed to have to deal with errors or downtime at some point.