The uses honestly vary too. Dari is primarily used for official communication, high level speeches, and when conferring anything among the educated withing Afghanistan. At field grade and higher levels, knowing Dari is a LOT more practical as a base. The more educated will know and understand Arabic (typically Mullahs, Mulawei), but there’s a pretty wide spectrum of those who can understand and regurgitate some of the fusha found in the Quran, and those who actually can converse in Arabic.
As the company level and lower, odds are the shared Dari won’t be worth nearly as much as a fairly basic wide Pashto/Pakhto base, despite a lot of the differences that can exist village to village in mountainous areas, the Pashto will absolutely be the most useful. For Sigint, Pashto and some Dari is what we need, but instead we get a lot of Farsi/Dari speakers who can rely on shared vocabulary and some syntax understanding to get by, but the net result on our end is mediocre translation without the military understanding of what kind of conversations are going on - a military tactical linguist with limited training is worth 4x as much as the contractors we get through MEP and pay 200k/yr for.
Whenever radio chatter consists of Arabic, it’s always something directly from the Quran (usually poorly translated, or used as coverterm). Valuable - yes, but outside my job I wouldn’t put much focus on it at all.
That Baluch/Baloch/Baluchi, and Brahui, as well as other lower density languages do exist down south (southern Helmand, southwestern Kandahar, and Pakistan out towards Girdi Jangle), but expertise and training is damn scarce, and most of our dealings with Baloch that far south in the Helmand valley, the guys we care about speak other languages anyway. I wouldn’t worry about it, since the good English/Brahui or other sorts of speakers are literally hard to find on the globe.
Further north Tajik may become practical, although the most exposure I got to that was the Hazara guys in the ANA, who all spoke very good Dari, and some Pashto, and even had a pretty good working understanding of Arabic.
Urdu may exist, but won’t be particularly useful, certainly nothing in the area that ISAF operates where spending resources learning Urdu should come before mastering Pashto, Dari, then Arabic and Tajik.
As far as the dialects and diversity of those - it’s absolutely ridiculous how fast dialects can diverge when the entire culture doesn’t revolve around written language, to the point where villages 5km from each other can have linguistic misunderstandings.