Of course, there's a chicken/egg thing here. Might as well say "it seems a bit extreme to include Python" just because the occasional app is written with PyGTK, for example.
There are plenty of folks writing applications in C# using the GTK bindings, which in turn need Mono. Tomboy is just one that's become popular enough that people want it to ship as part of Debian's distribution of GNOME (not the only one; IIRC there are three apps in there now which need Mono). So sooner or later somebody's gotta ship it by default to support an application, and then folks who don't have religious attachments to avoiding Mono will think it's no more unusual than any other shared dependency of commonly-used applications.
There are plenty of folks writing applications in C# using the GTK bindings, which in turn need Mono. Tomboy is just one that's become popular enough that people want it to ship as part of Debian's distribution of GNOME (not the only one; IIRC there are three apps in there now which need Mono). So sooner or later somebody's gotta ship it by default to support an application, and then folks who don't have religious attachments to avoiding Mono will think it's no more unusual than any other shared dependency of commonly-used applications.