I respect Stallman a lot. I’m a fellow of FSFE. His positions are usually well thought and argued, though I can’t say I agree with him on all of them, and I don’t agree with this one.
On the issue of the plain usefulness of Mono apps; well some of them are just excellent. I use f-spot for managing my photo collection and have to say no other application I’ve used comes close. Tomboy is itself a considerable improvement on the almost useless sticky notes applet that’s been in Gnome for some time; though I’ve yet to work out what minimum I need for ssh synchronization (to be fair, I’ve hardly tried).
And I should declare an interest. I thought Gnome-Do was absolutely excellent when I stumbled upon it, and I helped in a modest way to initially package it for Debian (specifically I updated the Ubuntu stuff for the plugins package and prepared that for its initial import to the archive), albeit I haven’t worked on it since. It’s fairly clear to me that implementing this in, for example, C++ looks like it could be a lot more work. I can’t say I’ve studied it in total depth, but that was my feeling when working with the code.
Anyway, never mind all the issues of sheer practicality; Jo Shields of the Debian Mono team, wrote an excellent rebuttal to some of the nonsense that was posted about Mono. It makes excellent reading. It is calmly delivered (with an undertone of quite justified and controlled anger), carefully argued and a cautionary tale about some of the nonsense we in the Free Software community can get wrapped up in, it’s value extends well beyond the current debacle.