The problem with such a low pH is that the nitrogen cycle slows down. Ammonia can build up because of this. In a low pH tank it's not a huge deal because Ammonia (nh3) converts to Ammonium (nh4+) which is far less toxic, but if something is added to the tank, or a big water change happens, that ammonium could shift back as the pH goes back up towards 7.
As long as it's working the way it is, I'd leave it be. But I do run crushed coral in the filter of my tanks with low hardness to help maintain stability.