chriton227
Explorer
If you go all utility, then casting stat doesn't matter. If your only purpose is to be a magical thief, you don't care about casting fireball or whatever, and since you don't have the room for it anyway it doesn't matter. A wizard will always have the temptation to wear another hat tomorrow, and has less credibility on going on a niche. This was skill intensive and needs int, but other niches don't -maybe int 13 for combat expertise for the gish-. That is why I love 3e sorcerers, they let you build your niche and be good at it.
But the casting stat does still matter. That 13 means that you can't cast 4th level or higher spells, plus starting at level 4 you will have fewer spells per day as a result of the lower number of bonus spells.
And as far as being "good at it", you have to go cross class to get the open locks, disable device, move silently, and hide, while at the same time having fewer skill points than a rogue and having to reduce your Dex to put points in Cha for your spell casting, so your skills will each be 2-4 points lower to start than an equivalent rogue. Instant Locksmith (if you DM allows non-core spells) will get your Open Locks almost up to what a same-level rogue will have, except you only get a few castings per day and the rogue can do it at-will. You also can't find any traps with a search DC higher than 20 because you lack the rogue's trapfinding ability. All this means that you will start off a worse thief than a vanilla rogue and will fall farther behind every level, while not bringing the sneak attack damage to combat that rogue would have, being more fragile than the rogue (lower HP, no armor, no evasion, etc.), and also not bringing the magical firepower that a sorcerer is capable of bringing.