Is it favorable of rather a hindrance to be compatible to POSIX and similar standards?
On the other hand, (mostly) complying to these standards, made a really big body of software just work without any (or just trivial) porting. Especially so for command-line programs, and libraries.
But: a large part of today's user programs are not written according to POSIX et al. low-level interfaces, but against GNOME, GTK+2, and other high-level frameworks and libraries. It may be a valid option to enrich these instead of striving for total POSIX compliance -- and the high-level programs (that is, their users) may not even notice this, but we would avoid a lot of overhead that comes with wrapping the Hurd interfaces to be POSIX compliant.