Why require SLOW_BUT_NO_HACKS for stubs?

[Isaac Dunham]

Hello,
I'm using musl as a libc, and have run into a number of times that
gnulib stopped build.
By defining SLOW_BUT_NO_HACKS, the software ended up working.
This is the documented behavior, but it doesn't seem like the right one:
if a stub is usable enough to allow using it, why shouldn't it be
available whenever there's no alternative?

Is there any reason not to merge the 
#else if SLOW_BUT_NO_HACKS
sections with the 
#else 
#error
sections, either with #pragma warn instead of #error, or without any
messages?

Isaac Dunham