You favour longer lasting spells, such as Stoneskin and Armor but also avoiding damage by not standing still. Mirror Image is a multiplier for Stoneskin, which sometimes protects the Mage even from multiple salvos of Arrows. There's also Protection from Missiles and Weapons. Plus, the Sorcerer's spontaneous spellcasting is amazing for this, given how many times you can cast Stoneskin while still leaving some points left for Greater Malisons and such, should you need it.
The Mage doesn't have to be a substitute for the Fighter, but if you play a no-rest run its good to make use of every resource at your disposal.
SoD has a lot of trash fights. Yes, it also has some fights with a dragon, lich, mind flayer, etc. I've never once said you couldn't tank with a mage. As Lhynn said, it's situational and fighters are good for trash mobs and Mages are good for others. I agree with that. To make a general statement for the IE games and in particular this game (BG and SoD) and say Fighters are sub par is false. It depends on the situation. The complaint I'm seeing is there's not much choice with Fighter types who can tank. There's Minsc at the beginning and who else? When do you get the other two fighter types (Khalid and Dorn) who can also be on the front line? Quite a distance in the game.
I would not use a mage for tanking trash mobs when you can give Minsc the Guantlets of Dex including some of the best armour and items in the game to lower his AC even further to avoid being hit. He already has 18/93 STR so he can hit hard. And he's also very hard to hit by trash mobs with his low AC. Then you don't have to use spells on your Mage to tank when you can use those fighter types for those trash fights. Again, that's been the complaint from people who would like more Fighter type tanks up front at the start of the game. I agree with them.
Have a look at Sensuki's IWD lets play videos. Does he use Mages all the time to tank up front? No, because there's a lot of trash fights and he doesn't need to. Was he potion and rest spamming? No. Oh he must be a noob then for not using a Mage to tank because Fighter types are sub par! Obviously that's not the case. And I'll say it a third time, the complaint is there's not many options at the start of the game to have fighter type NPCs to tank up front and the accusation is these people who use fighter type characters to tank are newbies. Some might be newbies, however it's a true and trusted method by a lot of players including those who aren't newbies. I've been arguing that it would have been good to have more of the original fighter type NPCs from the base game to have as a choice to either accompany Minsc or to use other original NPCs instead of Minsc from the start of the game. I think that would have been a great option. Why anyone would be against this completely baffles me.
And my comment about writing a few lines for those other NPCs and not having a story. I don't see a problem with that because I would rather have the original NPCs with some lines like they were in the base game than most of the original NPCS missing. I never asked for their life history, I'm asking them to accompany me. I don't see it as lazy for Beamdog to not write their life history because as an extension of the base game, it fits well that they don't tell me their life story. And some NPCs are quite secretive of their past and won't tell you. Just read their biographies. The argument that Beamdog didn't have time to write additional dialogue for every NPC in the game is a false one. If the option is to have all NPCs in the game but a lot won't have interjecting dialogue OR we cull a lot of the original NPCs from the base game to fit in 8 new romances (including NPCs that you've never met), I know which option I would take - and it's not the romances.
This game was supposed to bridge the gap between BG1 and BG2. What happened in between? Totally forget about some of the NPCs that were travelling with you. They've just upped and vanished. Instead, we get things like meeting a gay gnome that you can romance! Where the hell did that come from.