Well no offense Theymos but you really need to get some help with running the forum (not applying for the job here) because it's seriously under-administered. It should have a much bigger crew relative to the number of members. And I'm not just talking about mods, a good developer would be handy.
These are all very good points. Unfortunately, the original subject of the thread was deletion of off-topic comments, not the forum's administrative load. Under a strict interpretation of the forum's rules--the alternative being arbitrary censorship at the moderators' discretion--this perfectly reasonable comment must be deleted.
Personally, I would prefer to see off-topic replies merely hidden by default (with a show/hide toggle) rather than deleted outright, doubly so if the forum does not retain a copy of the deleted comment in the poster's profile. It's much easier to delete a comment than it is to get it back, and a comment which is off-topic can nonetheless be insightful and/or informative in a more appropriate context. In any case, the problem of off-topic replies is really more a reflection on the forum's lack of threading than any fault of the users.