[debatem1]

On legal and IP issues--

Some industries are under various requirements not to be user-modifiable. Some of those requirements are uncommonly applied or are uncertain (wifi routers), but some have serious teeth (export controlled munitions). For devices in those classes they often can't be open without risking liability.

On the IP side, some people really care about keeping firmware proprietary. Leaving JTAG in a mode that is meaningful for debugging that firmware will pretty much completely destroy that. So if you super duper care, you sometimes put clauses in your contracts holding the integrator or OEM responsible if your firmware leaks.

Regarding a race condition, yeah. It's pretty common for devices to come up open, then harden up-- and not just for JTAG. It's also not unique to the register-setting approach.