But, as the linked post points out, all those arguments fall by the wayside when the alternative is an in-house language used by no other production project on Earth. Developing your own language is not really "conservative" at all.
But the in-house language was basically a variant of Visual Basic. They basically created their own DSL because they didn't feel like any other tool satisfied their needs. Joel's not an idiot -- I'm sure he weighed the cost of doing his own language vs. the alternatives and found that the direction he took was the better way for the time.
It would have been more of a mistake if Fogbugz turned out to be a load of crap, but it's pretty good. I prefer it to JIRA.
