I'm still not completely seeing it. I can see that yes, spell slots can be more valuable than Arcane Ward ablative HP, but that doesn't just apply on the first hit of the day: it applies each and every time when you are hit and value your spell slot more than the HP you would preserve. If you have one 1st level slot left and you're saving it for HP, and you think you're going to rest as soon as you kill this last group of Gricks, you won't spend that spell slot on Shield even if the hit would take you down from 25 to 15 HP. That isn't unique to abjurors, it's just part of the tradeoffs of the Shield spell.
The only unusual thing about abjurors is that their Arcane Ward gains 2 HP each time they cast Shield, so if you Shield when you're at full Arcane Ward HP you lose those 2 HP. To me that still doesn't seem like a big deal, since you can recharge your Arcane Ward between encounters anyway with Alarm--and if you're in an area dangerous enough that you won't want to stop and do that, you're fairly likely to be in a high-intensity combat where that Shield spell you're foregoing (in order to gain 2 HP later on) would save you 10 or 20 HP right now, which is a pretty good tradeoff for one 1st level spell slot. In short, if I were playing an Abjuror, I probably wouldn't view the very first hit of the day as something special requiring special handling: instead, I would eschew Shield entirely when in low-intensity combats (even after the first hit), and use it liberally to stay alive in high-intensity situations.