It is one of those slightly "word of mouth" things. When FSX first came out a number of people found that when they tried repainting the default aircraft the textures would fail to appear. These all seemed to be done without mips and the suggestion to try again but with mips included seemed to fix all these cases

I admit I haven`t done any direct experimentation to confirm or deny this.

Like many texture things this may also be graphics-card dependant. Most of the DirectX work these days is done on the card hardware and different cards may treat the absence of mips differently. I recently had a case where a particular texture was actually crashing the sim on some machines and working perfectly on others.
It might also be format based - it may be that DXT1/3/5 must have mips while 16 and 32 bit don`t.

You may well be right and mips are not required but if you get some customers reporting untextured aircraft this might well be the cause.

And who knows what will happen when DX10 takes off...