I see two issues that a standard would help address:
- Portability. If a system design includes a third-party OEM module, then JTAG cannot be effectively applied to the system if the pertinent JTAG data for the module is not portable from the OEM vendor to the system builder.
- Tool support. Existing JTAG tooling provides support for some architectural features and devices that might be employed in a system JTAG design, but it is not comprehensive, meaning that designers need to be cogniscent of the tooling limitations. A standard, if it provides architectural guidelines, would help tool vendors to provide more focussed support, and allow designers to work reasonably safe in the knowledge that the design can be tested whatever test platform may be used.