I was asking myself the question. It's just that for coherency purpose with the refacotr of Input filter, it means we would need to do the same for input filter too. But hte object graph of values may be very large to serialize. I'm not sure this is a good idea?

Also, for jsonSerialize, this is handy for REST API, where we are interested only about error messages.

But indeed, this is something we must think about :).