Night vision was a solved problem in the 80s that BattleTech is the future of. It's absolutely bonkers that it notionally doesn't exist in 'Mechs.
In the '80s there night vision existed various sizes and applications, but it still had a host of problems in blooming, oversaturation, and narrow field of view. That's hardly a solved problem. Even with real-world modern night vision gear, driving around safely while using it is tricky. Night vision, even mixed display (digital overlay of optical, passive thermal, light intensification, and contrast/brightness diddling with LIDAR/mm wave radar input) does exist in BattleTech, and there will *still* be operational problems.
Put another, more rule-wise, way - if there's no changes between day and night, or weather... what's the point? It's there to address the many "Hey wait, shouldn't there be a benefit to attacking under the cover of darkness/snowstorm/whatever condition?" questions and "OK, here's how to handle that..." answers.
But in the end, it's all still BattleTech. You can do you, as you want to. I can do mine, as I see fit as well. I have reasons for the way I see things, and they're well founded. I'm only communicating how I see things and the reasoning behind it. I'm not insisting that you *must* play this way, or that it's somehow not BattleTech otherwise.