Leaving (semi)transparent lips or eyebrows allows a mod skin to change lips/eyebrows color using the SL sliders, BUT:
transparent areas will actually show the under default SL kin in those areas, and lips color and specially eyebrows color would be the SL default ones, that we all know won't look much good specially on a well made skin. You would't like to have the SL default eyebrows in the color you chose ON a 4k skin will you?

The only way to have nice eyebrows that fit a particular skin, is that skin-maker makes different "face tattoos" of same skin tone with different high detailed eyebrows in diffferent colors.
Pratically that means different skins, one for each eyebrow color.
There is no way you can have your eyebrows on a different skin, even if the skin would be mod, unless you ask the skin-maker to make a skin that looks like you want ^^
The reason many skin-makers set their skins to no mod, is not cause they want to limit you, is just that often a skin is a complete tattoo (well 3 tattoos, face, upper body and lower body) and even if it's mod sliders won't do any change on it.
On the other hand, leaving skins semi transparent (specially in some parts like lips) would let you mod color, freckles and other things using the sliders, with the limitation that what you see under the semitransparent area is the DEFAULT SL skin. That will make the original skin lose some of its features (depending on how much transparent the skin is).
So the best skins are NOT transparent, s they need to show the total custom work studied and designed by the skin-maker ^^
And thus the mod rights are useless (and dangerous, cause if you by mistake change the tattoos you'll lose the textures) and that's wy most skins are no mod.
The best thing you can find is that skin-makers do many different skins with different eyebrow colors so you can chose wich one suits you better