So, Ive built my two main PCs around fire/earth mage and sneaking archer. Since Jaha (the first NPC that joined me) is a good crafter from the start Im thinking of skipping crafting altogether on my main characters. The question is, do I really keep Jaha all the way through or does he suddenly levae me after all Ive invested in him (Im thinking about what sometimes happened in old Baldurs gate when some NPCs left you and never returned, taking a bunch of stuff with them and leaving a hole in the party setup).

In a non-spoilerish way, you should be able to keep any NPC if keeping them is your goal. Even if you ask them to leave nicely using the dialog, they will go wait and tell you where and will rejoin when asked. Other things that you can do to get them to leave on hostile terms are obvious and they give warnings. You should be able to tell.

I always keep Jahan. I use him for blacksmithing and crafting and as hydro, aero, witch mage. Since I start with a battlemage that I remove witchcraft in the creation screen Jahan gets that skill. He gets enough gear that I don't have to put more than one point each into the black/craft skills to max them.