Here is a little argument in support of the category of supererogatory actions:
1. Some good actions are praiseworthy.
2. No obligatory actions are praiseworthy.
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3. Some good actions are not obligatory.
Since by definition a supererogatory action is one that is good but not obligatory, the above amounts to an argument for supererogatory actions. The argument is valid and the first premise self-evident. So the soundness of the argument rides on the second premise. Here, I suppose, an appeal to intuition is unavoidable.
So I will simply state that it is morally obtuse to praise someone for doing what he is obligated (whether legally or morally) to do. You don't praise a person for driving in accordance with the traffic laws, you blame him for failing to do so. It is a sign of moral confusion and moral decline to praise people for doing what they ought to do.
Praise and blame attach to the supererogatory and the prohibited, respectively. Neither attach to the obligatory.
While I'm on the topic of moral confusion, I saw an official sign on a pedestrian traffic signal in Tempe, Arizona. It read: Please do not cross against the light. The pertinent code was cited. Now what moral idiot formulated this sign? 'Please' is a word used in making a request, not in issuing a command. Am I being requested to not cross against the signal? If it is a request, then it is permissible for me to decline. "No thank you, I prefer to do as I please regardless of public safety and order." And if it is a request, why cite the traffic code? Or am I being commanded to not cross against the signal? But then 'please' is out of place.