The standards bodies can encourage FRAND licensing, but there is no legal requirement backing it beyond the patent holder agreeing to it.

Thanks for the correction, I misremembered that, it seems. On the other hand, an SEP holder not licensing the patent with FRAND terms will have a hard time to establish any kind of standard, in some scenarios.

I agree with you, it would be counterproductive. The more common occurrance is that one of the patent-holding entities opts out of the FRAND agreement, holding all the implementors of the standard hostage. An example would be Forgent's acquisition of a patent they interpreted to be essential to JPEG. The patent was eventually invalidated in the courts, but it caused a lot of headaches for a few years in the early 2000s.