Barring people from using their chosen fonts inhibits me from using a quick crank heuristic to save my time by ignoring them, so I would be against it. We should let them select themselves out of the memepool.
On Less Wrong, I guess this heuristic mainly ignores people who wrote their article in MS Word and have not lurked here long enough to be aware of this pitfall. Is that strongly correlated with low quality articles?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.