Linux kernel as a Library (LKL)[0] uses nommu mode. I think mostly just because it was easier to get working fast than trying to emulate an MMU. They've attempted to upstream a patch to XFS to work in nommu mode[1], but met some resistance from some XFS developers.

[0]: https://github.com/lkl/linux

[1]: https://lkml.org/lkml/2015/11/17/862