The last company I worked at was pretty flexible with using whatever programming language made sense for the job, as long as everyone working on that project was comfortable with it. As far as I'm aware, they've kept this attitude to this day.
The biggest downside would appear to be the risk that new developers won't be able to contribute to a project, but I've found this isn't really an issue; most developers that I've met are pretty excited by the chance to use a new language. That excitement may also offset the inertia from using a new language.