It's more correct to say that non nocturnal monsters can't use their ranged attacks to hit the backline directly at night, not that it disables ranged attacks. The distinction is important, because if a mechanic DID disable ranged attacks in truth, you'd have to worry about retaliation on squishy ranged attackers, which could be horribly bad.
As for the defense reduction, it sounds ugly in theory, but in practice, as long as you do proper division building, it won't matter much, because what really matters is stuff like Hardy/Godly physique and other forms of mitigation that you have on your tank, which should be the only unit taking damage in a properly built division. At least from direct attacks.
I don't remember if Defense interacted with spell damage, or tactical skills and whatnot... but I think it didn't...? It's one of the reasons why spell barrier is one of the most common barriers you want on a division. Self destruct definitely ignores defense, so that's another thing where defense loss just doesn't matter.
Realistically defense only seriously matters on enemy units and specifically on higher difficulties, where stats are super inflated.
In short, don't worry too much about night/day, they have little relevance outside of units spawning most of the time as long as you have well organized divisions for your level.