SPARC: why it's not registerised anymore?

Folks, just out of curiosity I'm building latest GHC on SPARC/Solaris and I've noted that current GHCs (7.6 and HEAD) are building unregisterised by default. In the history I remember at least some GHC were registerised and GHC source code also (still) contains SPARC NCG so I'm curious what was the reason behind the switch? Any bug still waiting for a fix? Or just code rotted in a way it's not working any more? Thanks! Karel