Has anyone else noticed this - if you have a magic type follower like Brelyna she will start using a hunting bow with iron arrows - I think it might be because there's a separate bug that gives you the arrows you've been shot with (doesn't make that much sense really but that's why if your horse dies you'll sometimes find it has a few arrows of different types) and when she's got that she won't use her magic anymore. I ditched Brelyna for this reason and went and got Ilia - and guess what? Within 5 minutes she's forgotten her magic training and is only interested in her bow and arrows. Worse, when she asked to be made my steward, I could no longer ask her to follow me so I can't get the items I've given her back without pickpocketing or killing her! Geez... This game.

Are you saying the magic user followers use the bow all the time? I've not experienced this personally, but it could be because I avoid giving them any bows and arrows to use.

Magic user followers only have a limited pool of magica, the same as the player does. What could be happening, is that when they run out of magicka, they have to use any other form of weapon to hand. For this reason, I always give magic user followers a sword and shield to use as well. You can give followers 'magicka potions', so when their magica drops low, they will use these potions to replenish their magicka and continue using spells. Sometimes they will also use potions that fortify their magicka level, and magicka regeneration rate. Also, give them several magic staffs to use, and they use them and swap between them when they run out. In fact, if you give them a sword, shield, staffs and magicka potions, you will find that they will swap between all of these weapons depending upon how close enemies are, and how much magicka they have left.

Illia has a bug which prevents the player getting her to follow you again, after you have saved/loaded, and then dismissed her. Essentially, you can recruit and dismiss her repeatedly, until you save and load the game. After that, her option to recruit her again (after dismissal) vanishes. Yes, this is a particularly annoying bug.