I see advantages of getting JSON response and formatting in client side but are there any advantages by using JSON for form submission compared to normal submission?
One thing comes to mind when dealing with POST data is useless repetition:
For example, in POST we have this:
partners[]=Apple&partners[]=Microsoft&partners[]=Activision
We can actually see, that there is a lot of repetition here, where as when we are sending JSON:
{"partners":["Apple","Microsoft","Activision"]}
59 characters versus 47. In this small sample it looks miniscule, but these savings could go up and up, and even several bytes will save you some data. Of course, there is the parsing of data on server side, which could even out the differences, but still, i saw this sometimes helping when dealing with slower connections (looking at you, 3G and EDGE).