I don't see any reason not to blow money on magic arrows, TBH.
Basically, there's not that much to spend money on in the game. By the time you hit Level 4 you're going to have a big pile of money, but most of the vendors won't have any magic weapons/armors worth much. There's a real huge amount of magic amulets in the game as well, but almost no magic rings for some reason. Really besides healing pots, these arrows are just about the best thing you can buy...unless you want to buy some scrolls I suppose, but since there's no scroll scribing, I don't see the use.
In addition, in most cases, you can get the money back. In my current (good) run, I killed the trader of that cultist group hunting Karlach, and the two traders in the Goblin camp, getting back all my gold. I presume in an evil run you can kill the traders in the Druid camp.
You can have your wizard learn all the spells including the cleric ones. So there is technically a use for having them at least for your wizard to have access to all the spells. Heck if you wanted you could just have two wizards and no cleric eventually. I wouldn't advise it, but you could. Funny thing, which I am sure you are aware of. You can sell to that Zhent then kill him and reloot the loot you sold to him just to resale it back at the good camp. So it really does pay to be "good".
