Something that I often see people on the forums saying is, never choose a thief because thieves are only necessary for Find Traps and you always have a thief companion. While others say, always make your main character your thief because you're always going to need a thief.
Thing is, there's many optional uses for thieves, like setting traps, using stealth, backstabbing, and thief HLAs (including Use Any Item), that there's a reason you might want to have more than one, or at least have a character who can level up the class and won't permanently leave midway through, and the game just doesn't give companions suitable for that.
Also, one strategy I once saw in a speedrun is to do a mage to thief dual class, and I believe the player spent all the thief skill points on pickpocket, allowing the player to steal some items. (Note that I wouldn't recommend this for casual play, as your HP will be worse than what a thief would get.)
(There's also the fact that, personally, I'd like the main character to be a magic focused character, preferably one who can heal, and there being a thief shortage doesn't help.)
Lastly, I guess it's pretty obvious that there is no sorcerer companion, but I feel that any arcane caster you can create will be crappy compared to Edwin so I dunno.
The ability to spontaneously cast spells can be quite handy. If you get dispelled, for example, you can easily just put those spells back up. If you find that you chose the wrong spell for the situation, you can spend those slots on other spells without having to rest. If you use Limited Wish to recover spells, or use Simulacrum to create a duplicate, you don't need to worry about which spells are restored or are available to the clone. Also, multiple clones can spend their spell slots differently.
Also, Sorcerers can get rarer spells more easily. Because, for whatever reason, Mages don't get to choose new spells at level up, they're at the mercy of the game to provide scrolls for the spells you want; Sorcerers, on the other hand, aren't bound by this. A Sorcerer can, for example, learn Wish without needing to go deep into Watcher's Keep or reach the main Throne of Bhaal campaign.
There's also the fact that Edwin has only 9 Wisdom, making him a poor choice for casting Limited Wish or Wish. Furthermore, he's vulnerable to Holy Smite, which can be a problem if you want to use that spell offensively, and are relying on the fact that it won't hit non-evil targets in a game where most of the enemies are evil.