Spellthief? Fax, I can't believe you're suggesting that. A core only Rogue 3/Wizard 5/Arcane Trickster 10 has more Sneak Attack, access to much more and higher level spells, and comparable Skills.
I have to say though, I'd love to see someone actually try and play a Spellthief some day. It'd be a huge challenge, worse then playing Dan in Street Fighter.