White LED spectra contains quite a bit of blue range wavelength, which is bad for night vision. Red light is much better.
The eyes have cone receptors for day time and rod receptors for night vision. The rods contain rhodopsin, which enables night vision. It takes about 1/2 hour for enough to build up to have full night vision.
Exposure to shorter wavelengths quickly breaks down rhodopsin, which is why blue light is to be avoided.
Natural sunlight is strong in blue (and UV) spectra. Incandescent tungsten filaments have much less blue/green spectra, and a lot more orange/red. So it may be a bit "old school" to shun LEDs.
Newer OLEDs have spectra closer to candles and incandescent.
One interesting measure is how much the light suppresses melatonin, which helps one sleep. Newer OLEDs were found to be only fractional of blue light, so it would be important to be sure any replacement map light is appropriate for the task.
Quite simply because a random LED bulb is not certified for operation in an aircraft, and the light holder is not certified to operate with a LED bulb.
In practice it would probably work fine, but it is not out of the question that a LED bulb (sometimes containing internal SMPS power supply) might emit electrical noise, or perhaps even fail in ways that a normal light bulb wouldn't.
My interpretation of the comment was that replacing the bulb with an LED is going to draw attention to the fact that you have broken the rules. If you are going to go rogue on a minor thing like this, give the inspector some plausible deniability.
Sort of like if you buy some custom aftermarket taillights for your car that maybe aren't officially DOT approved. If you buy red ones the cop probably won't notice and wouldn't care much anyway (unless you're a jerk or have a dozen other equipment violations). If you buy green ones, the cop won't be able to overlook it or let you get away with it.
The other answers here about night vision etc. I think are good advice also.
Physical reasons for not changing to LED include
- Lower heat output - which sounds good unless there's something else dependent on the warmth. LED traffic signals in cold locations require heaters to melt ice, whereas the old 420V incandescent lamps keep themselves warm and ice-free.
- Lower power draw means less current - again plausibly good, unless there's something measuring the current. Example - indicators (blinkers) in a car may not cycle between on/off if they are LEDs The unit that does the blinking in older cars is thermal, and depends on the current flow to switch.
- Insurance - In the event of a claim, an insurer will take every possible option to reject the claim. If you've got anything out of specification the insurance adjuster will look for it, and potentially not pay out. Doesn't matter if its your insurer or the other person's insurer. And they know what to look for, its their job to find ways to not pay on claims.