That is a good point in that it is a very accurate reading of the JSON spec. In practice many (even most) JSON implementations don't give applications access to any precision beyond what an IEEE double can represent. So while you may take advantage of arbitrary precision in JSON and be fine according to the spec, your users will probably suffer data loss unless they are very picky about what JSON library they use. For example, JSON.parse() in JavaScript is out.
It's more than just precision, it's making sure that the same value comes out that went in, and that things haven't been subtly altered via unintended conversions between decimal and binary floating-point representations. Obviously this is quite important when you've got both text and binary formats.
Some applications really need decimal values, and some really need IEEE floats. Ion can accurately and precisely denote both types of data, making it easier to ensure that the data is handled properly by both reader and writer.
