Venturer and Nightblade both have this problem that fractional arcane casting kinda blows. Nightblade is OK during the dungeoneering levels, where he can contribute usefully with Sleep, but when wilderness levels roll around, he is kinda SOL, because he doesn't get fireball until 9th level. His magic no longer contributes effectively. On the flip side, the Venturer gives up a whole classpoint for a rapid casting progression starting at 8th level... but he's running on 3 points for most of the adventuring range, and by 8th level Charm Person has stopped winning encounters. Granted, Venturer is an Arcane 1 class, but the problem still stands that delaying the onset of casting is really terrible during the levels before it kicks in, and then isn't very useful after either.
What if... we gave half-casters access to spells that solve problems appropriate to their level range, but not very many of them? Like, half as many as a mage of their level, and also halve bonus repertoire slots from Int. Maybe round spells per day up, and repertoire slots down. So they don't get many spells per day, and also don't have a whole lot of options to use them, but they always have something useful in their current tier of play. They will never be as good as the wizard at the wizard's job, but they also won't ever be "wasted points" either.
Under this system, a 6th-level nightblade with 16 Int gets 1 1st, 1 2nd, and 1 3rd-level spell per day, and gets two known spell at each of those levels, whereas the same character as a mage would have 2/2/2 and 4 spells known at each level. Sounds about worth half a caster, no? And this "worth about half a caster" remains true across the level range. They're worth a little more at odd levels, because that one spell of the new level is more than half what the mage gets, but their reduced repertoire from Int (combined with their increased multi-ability distribution relative to mage) means that they are less likely to know any given useful spell. One exception is 1st level, where the first spell is judge-selected, typically Sleep or Charm Person. As a judge, I would not feel bad about making a half-wizard roll his spells at 1st level rather than guaranteeing an encounter-win spell, because they have ways to contribute besides magic (maybe add a "reroll really terrible spells" option. The other option, rounding down at odd levels, makes the half-mage substantially less useful than half of a mage a lot of the time, especially in high-wilderness levels where being one level behind is actually a lot of XP and time spent without current spells. It also denies them any casting utility at 1st level. As a result, I think I'm happy with round spells up, round repertoire from Int down.
A reasonable justification for this system is that half-casters keep up with full-casters on arcane theory and esoteric lore (hence comparable max spell level), but do not devote the same amount of time and resources to rituals for maintaining more repertoire slots and tapping more raw power, because half-casters are busy doing other stuff (like backstabbing people).