I was reading Wikipedia, and it maintains that it's unusual for a language to have a voiceless-voiced-breathy distinction (without a voiceless aspirated), but that the Sanskrit 4-way distinction is less weird. I've also heard of the glottalic theory from other sources, which is supposed to make PIE's phonology less weird.
I'm surprised that voiceless-voiced-breathy is so weird because it seems like the natural result of a merger in an aspirated-tenuis-voiced-breathy distinction. As I understand it, the difference between aspirated, tenuis, and voiced is mainly voicing onset time with tenuis being between the other two. Isn't it natural that tenuis should merge with one of the other ones? (maybe losing aspiration afterward?) Breathy voiced is so different it makes sense to me that it would stay different.
I suppose it's just an empirical fact that this distinction is uncommon, but does anyone know why it's so uncommon? (It's not just because so many languages are descended Proto-Indic is it?)