The base class spellthief gains at level 1 the supernatural ability steal spell that, in part, says
A spellthief can siphon spell energy away from his target and use it himself. A spellthief who hits an opponent with a successful sneak attack can choose to forgo dealing 1d6 points of sneak attack damage and instead steal a spell, or the potential to cast a specific known spell, from his target. If the target is willing, a spellthief can steal a spell with a touch as a standard action.
The target of a steal spell attack loses one 0-level or 1st-level spell from memory if she prepares spells ahead of time, or one 0-level or 1st-level spell slot if she is a spontaneous caster. A spontaneous caster also loses the ability to cast the stolen spell for 1 minute. If the target has no spells prepared (or has no remaining spell slots, if she is a spontaneous caster), this ability has no effect. A spellthief can choose which spell to steal; otherwise, the DM determines the stolen spell randomly. If a spellthief tries to steal a spell that isn’t available, the stolen spell (or spell slot) is determined randomly from among those the target has available.…
After stealing a spell, a spellthief can cast the spell himself on a subsequent turn. Treat the spell as if it were cast by the original owner of the spell for the purpose of determining caster level, save DC, and so forth. A spellthief can cast this spell even if he doesn’t have the minimum ability score normally required to cast a spell of that level. The spellthief must supply the same components (including verbal, somatic, material, XP, and any focus) required for the stolen spell.
Alternatively, a spellthief of 4th level or higher can use the stolen spell power to cast any spellthief spell that he knows of the same level or lower (effectively, this gives the spellthief one free casting of a known spell). A spellthief must cast a stolen spell (or use its energy to cast one of his own spells) within 1 hour of stealing it; otherwise, the extra spell energy fades harmlessly away. As a spellthief gains levels, he can choose to steal higher-level spells.… (Complete Adventurer 16)
(Emphases mine.) My concern is this: It seems like when a spellthief uses steal spell against a caster that casts spells without preparation (like a sorcerer) that the spellthief only steals the spell slot and not the knowledge of the spell itself. In other words, this description doesn't seem to mention, for example, that the spell that the victim temporarily lost the ability to cast also temporarily becomes for the spellthief a spell known.
Thus, prior to attaining 4 levels in the spellthief class, a spellthief that swipes a spell slot from, for example, a sorcerer can't actually do anything with that swiped spell slot. Sure, the sorcerer loses a slot and for 1 min. can't cast the picked or randomly determined spell, but that seems awfully unsatisfying. I mean, that makes, in this case, the low-level spellthief less of a spellthief and more of just a spellvandal.
Likewise, after attaining 4 levels in the spellthief class a spellthief that swipes a spell slot from a sorcerer can only use that slot to cast the spellthief's own spellthief spells and never the sorcerer's own temporarily-lost spell known.
Is my reading of the rules here accurate?
Note: To be totally clear, because the steal spells ability runs over 700 words, I cut examples and other extraneous information from the description above. (Game Designers: If it takes over 500 words to describe a class feature, it's probably overcomplicated! Break! It! Into! Pieces!) Fortunately, as mentioned, the spellthief class is available online. If answers want to bring into the conversation material that I cut, that's totally acceptable. I'm looking for an answer not an argument.
Also, once again, I know I'm late to the party and this was likely hashed out on the long-lost-and-lamented Wizards of the Coast forums back in the day, so answers that link to archived discussions are not only totally acceptable but also welcome. An answer needn't present new information to be good.
Finally, yes, for an upcoming minicampaign, I'm considering playing a spellthief. The campaign'll largely use the E6 rules, and that environment seems ideally suited to the spellthief's otherwise limited trick repertoire.