The author put mongrel out there under a free license, as well as some unnamed gem, and various other things. The gem is not included in Debian. As to the "benefits" of Debian including anything, they certainly aren't pecuniary, but accrue mostly to the end users (or not, in this case). So, if you help Debian fix something, the beneficiaries are ultimately all the users of Debian, not really 'Debian' itself.

> Mac OS X - Fine. FreeBSD - Fine. RedHat/CentOS - Fine. SuSE - Fine.

Any sufficiently complex system, including every one of the above, has problems. They may differ from Debian's in type, nature and even quantity, but they exist. Including missing commands, disabled functionality, and misconfiguration.