In my experience, and just from a practical point of view, I found it useful for "production" images to have here Error instead of MAError, since even in case of an unexpected error, Pier keeps handling requests gracefully, while rendering the error message on the client side. For development images, it would be sufficient to add a 'halt' inside the exception handling block.
I see your point. My understanding for the intent of the code was as a mechanism to display form validation errors, rather than general programming errors. I had a couple of bugs when I ported to Gemstone that were silently consumed (the errors weren't displayed) by PRContentsWidget's error handling and felt that this wasn't the intention of the original code. It seemed to me that programming error exceptions should be propagated and handled in a more general exception handler.
In the case you mention, where 'Pier keeps handling requests gracefully' would the errors make any sense to the user and could they take remedial action to correct the error?
Nick