The point of unseen seer is that it gives full spell progression.
It also gives 6+int skillpoints, a good class skill list for a rogue, and occasional sneak attack and not terrible HD and BAB.
It gives you the options and utility of a caster while remaining definably a 'rogue'. I've played rogue/wizard/unseen seers at both high and low level, and they 'feel' like a rogue while keeping up with the 'big boy' spellcasters. Daggerspell mage doesn't really do that. It's okay at low levels because you can use touch spells with your full attack, but the lost caster levels really add up quickly and you lose puff fairly fast.
As for Duskblade, at level 5 it's kind of neat, you're shocking grasping and hitting people with a weapon at the same time. At nearly any level beyond that, it's very weak as your blasting and melee potential are both very sub-par. It also doesn't really feel 'roguelike'. If you want duelist style dagger fighter, that's different, and honestly i'd be looking at Swordsage/Warblade and Jack B Quick/nausea lockdown builds, as opposed to anything else.
1 level of Spellthief and the Master Spellthief feat is something you can add on top of anything castery. It's not really a base build, it's more of an optional addon if you have a level you don't really need for anything else.
Illusion spells typically don't care about caster level at all except for duration, which isn't often an issue (usually you break your invisibility by sneak attacking, or get past whatever you were stealthing past, before it becomes an issue, or the illusion lasting for 10 minutes longer doesn't matter as they would find out anyway when it broke that the macguffin is gone). Divinations do because of nondetection, which is defeated by a Caster Level Check. It's not a huge point, you're really not going to notice -1 CL or +1 CL much either way, even if you're blasting, 1d6 is 3.5 damage on average, and if you're an unseen seer, you shouldn't be blasting, you should be buffing yourself and then murder-daggering people.
Either way, don't forget Darkstalker, and pack lots of scentblind oil, and when in doubt, run away (rogue style).